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what is the difference between genotype and phenotype?
-genotype refers to the genes or the set of alleles found within an individual
-phenotype refers to the manifestation of a particular character or trait
what is the principles of segregation?
the principle of segregation, or Mendel's first law, states that an organism possesses two alleles of any one particular trait and that these alleles separate during the formation of gametes
-one allele goes into each gamete
-explains how the genotypic ratios in the haploid gametes are produced
how are Mendel's principles different from the concept of blending inheritance
Mendel's principles assert that the genetic factors or alleles are discrete units that remain separate in an individual organism with a trait encoded by the dominant allele being the only one observed if two different alleles are present. According to Mendel's principles, if an individual contains two different alleles, then the individual's gametes could contain either of these two alleles(but not both)
blending inheritance proposes that offspring are the result of blended genetic material from the parent and the genetic factors are not discrete units. once blended, the combined genetic material could not be separated from each other in future genetations
what is the concept of dominance? how does dominance differ from incomplete dominance?
the concept of dominance states that when two different alleles are present in a genotype, only the dominant allele is expressed in the phenotype.
Incomplete dominance occurs when different alleles are expressed in a heterozygous individual and the resulting phenotype is intermediate to the phenotype of the two homozygotes
what is the addition and multiplication rules of probability and when should they be used?
the addition and multiplication rules are used to predict the ratios of offspring in genetic crosses
-the multiplication rule allows for predicting the probability of two or more independent events occurring together. according tot he multiplication rule, the probability of two independent events occurring together is the product of their probabilities of occurring independently.
-the addition rule allows for predicting the likelihood of a single event that can happen in two or more ways. It states that the probability of a single mutually exclusive event can be determined by adding the probabilities of the two or more different ways in which this single event could take place. The multiplication rule allows us to predict how alleles from each parent can combine to produce offspring, while the addition rule is useful in predicting phenotypic ratios once the probability of each type of progeny can be determined
what is the chromosome theory of inheritance?
the theory states that genes are located on the chromosomes. The independent segregation of pairs of homologous chromosomes in meiosis provides the biological basis for Mendel's two rules of heredity
what is the principle of independent assortment? how is it related to the principle of segregation?
according to the principle of independent assortment, genes for different characteristics that are at different loci segregate independently of one another.
the principle of segregation indicates that the two alleles at a locus separate; the principle of independent assortment indicates that the separation of alleles at one locus is independent of the separation of alleles at other loci
in which phases of mitosis and meiosis are the principles of segregation and independent assortment at work?
In anaphase I of meiosis, each pair of homologous chromosomes segregate independently of all other pairs of homologous chromosomes. The assortment is dependent on how the homlogs line up during metaphase I. This assortment of homologs explains how genes located on different pairs of chromosomes will separate independently of one another.
Anaphase II results in the separation of sister chromatids and subsequent production of gametes carrying single alleles for each gene locus as predicted by Mendel's principle of segregation
supposed that you are raising Mongolian gerbils. You notice that some of your gerbils have white spots, whereas others have solid coats. What types of crosses could you carry out to determine whether white spots are due to recessive or dominant allele?
if white sports are recessive, then any gerbil with white spots, then any gerbil with white spots must be homozygous for white spots(ww) and a cross between two white spotter gerbils(ww x ww) should produce offspring with only white spots.
If white spots are dominant to solid then a cross between a gerbil with white spots and a gerbil with a solid coat should produce either progeny all having solid coats(WW x ww=Ww) or a progeny where one half have solid coats and the other half have white spots (Ww X ww= 1/2 Ww x 1/2 ww)
hairlessness in American rat terriers is recessive to the presence of hair. Suppose that you have a rat terrier with hair. How can you determine whether the dog is homozygous or heterozygous for the hairy trait?
H= dominant(hair) allele
Because H is dominant to h, a rat terrier with hair could be either homozygous(HH) or heterozygous(Hh). To determine which genotype is present in the rat terrier with hair, cross this dog with a hairless rat terrier(hh). If the terrier with hair is homozygous (HH), then no hairless offspring will be produced. If the terrier is heterozygous(Hh) then we would expect one-half of the offspring to be hairless
describe the XX-XO system of sex determination. In this system, which is the heterogametic sex and which is the homogametic sex?
in the XX-XO sex determination system, females have two copies of the sex determining chromosome whereas males have only one copy. Males must be considered heterogametic because they produce two different type of gametes with respect to the sex chromosome: either containing an X or not containing an X
what is the pseudoautosomal region? how does the inheritance of genes in this region differ from the inheritance of other Y linked characteristics?
the pseudoautosomal region is a region of similarity between the X and Y chromosomes that is responsible for pairing the X and Y chromosomes during meiotic prophase I. Genes in this region are present in two copies in males and females and thus are inherited like autosomal genes, whereas other Y linked genes are passed on only from father to son.
how is sex determined in insects with haploiddiploid sex determination?
diploid individuals are female, whereas haploid individuals are males. Eggs that are fertilized by a sperm develop into females, and eggs that are not fertilized develop as males
how does sex determination in Drosphila differ from sex determination in humans?
in humans the presence of a function Y chromosome determines maleness. People with XXY and XXXY are phenotypically male.
in drosophila, the ratio of X chromosome material to autosomes determines the sex of the individual, regardless of the Y chromosome. Files with XXY are female and flies with XO are sterile males
give the typical sex chromosomes found in the cells of people with Turner syndrome, Klinefelter syndrome and androgen insensitivity syndrome, as well as in poly-X-females
Klinefelter syndrome-XXY (rarely XXXY or XXYY)
Poly X females-XXX(rarely XXXX or even XXXXX_
what characteristics are exhibited by an X linked trait?
males show the phenotypes of all X linked traits regardless of whether the X linked allele is normally recessive or dominant. Males inherit X linked traits from their mothers, pass X linked traits to their daughters, and through their daughters to their daughters' descendents, but not to their sons or their sons' descendents.
explain how Bridges's study of nondisjunction in Drosophila helped prove the chromosome theory of inheritance
Bridges showed that in crosses with white eyed flies, a sex linked trait, exceptional progeny had abnormal inheritance of sex chromosomes. In matings of white eyed females with red eyed males, most of they progeny followed the expected pattern of white eyed males and red eyed females. Exceptional red eyed male progeny were XO and exceptional white eyed females where XXY. These karotypes were exactly as Bridges predicted with his hypothesis that the exceptional red eyed males inherited their X chromosome with the red eye allele from their red eyed fathers and were male because they did not inherit an X chromosome from their mothers, resulting in an XO condition that is phenotypically male but sterile. Moreover, te exceptional white eyed females inherited two X chromosomes from their white eyed mothers as a result of nondisjunction in meiosis I of the female, and none from their red eyed fathers, receiving instead a Y chromosomes to make then XXY females. Bridges linked exceptional inheritance of a sex linked trait to exceptional inheritance of the X chromosomes: the linked exceptions proved the rule that genes reside on chromosomes.
explain why tortoiseshell cats are almost always female and why they have a patchy distribution of orange and black fur
tortoiseshell cats have two different alleles for an X linked gene: X+(non orange or black) and Xo(orange) The patchy distribution results from X inactivation during early embryo development. Each cell of the early embryo randomly inactivates one of the two X chromosomes and the inactivation is maintained in all of the daughter cells. So each patch of black fur arises from a single embryonic cell that inactivated the Xo and each patch of orange fur arises from an embryonic cell that inactivated the X+
what will be the phenotypic sex of a human with the following gene or chromosomes or both?
XY with SRY gene deleted-FEMALE
XY with the SRY gene located on an autosomal chromosome-MALE
XX with a copy of SRY gene on an autosomal chromosome-MALE
XO with a copy of SRY gene on an autosome-MALE
XXY with the SRY gene deleted-FEMALE
XXYY with one copy of the SRY gene deleted-MALE
in humans, a single functional copy of the SRY genes, normally located on the Y chromosome, determines phenotypic maleness by causing gonads to differentiate into testes. In the absence of a functional SRY gene, gonads differentiate into ovaries and the individual is phenotypically female
in certain salamanders, the sex of a genetic female can be altered, making her into a functional male; these salamanders are called sex reversed males. When a sex reversed male is mated with a normal female, apporximately 2 of the offspring are female and 1 are male. How is sex determined in these salamanders.
the 2:1 ratio of female to males could be explained if a quarter of the progeny are embryonic lethals. The sex reversed male has the same chromosome complement as normal females. If females are homogametic (XX), then matings between sex reversed males(XX) and normal females(XX) must result in all XX female progeny, with no obvious reason for embryonic lethality. However if females are heterogametic(ZW) then ZW(sex reversed male) crossed to ZW(normal female) results in 1/4 ZZ(male), 1/2 ZW(female), and 1/4 WW(embryonic lethal). The net result is 2:1 ratio of females to males
in organisms with the ZZ-ZW sex determining system, from which of the following possibilities can a female inherit her Z chromosome
her mother's mother-NO
her mother's father-NO
her father's mother-YES
her father's father-YES
females inherit the W chromosome from the mother and the Z chromosome from the father. If we exclude the possibility that the mother and father are sibilings of the same paternal line, then no Z chromosome can be inherited from the mother's parents. Males inherit one copy of the Z chromosome from each parent, so females have equal probability of inheriting the Z chromosome from the father's mother or the father's father
Red green color blindness in humans is due to an X linked recessive gene. Both John and Cathy have normal color vision. Cathy gave birth to a color blind daughter. John filled for divorce, claiming he is not the father of the child. Is John justified in his claim of nonpaternity? If Cathy had given birth is a color blind son, would John be justified in claiming nonpaternity?
Since color blindness is a recessive trait, the color blind daughter must be homozygous recessive. Because the color blindness is X linked, then John has grounds for suspicion. Normally, their daughter would have inherited John's X chromosome. Because John is not color blind, he could not have transmitted a color blind X chromosome to his daughter. A remote alternative possibility is that the daughter is XO, having inherited a recessive color blind allele from her mother and no sex chromosome from her father. In that case, the daughter would have Turner syndrome.
If Cathy had a color blind son, then John would have no grounds for suspicion. The son would have inherited John's Y chromosome and the color blind X chromosome from Cathy
red green color blindness in humans is due to an X linked recessive gene. A woman whose father is color blind possesses one eye with normal color vision and one eye with color blindness
a.propose an explanation for this woman's vision pattern
b. would it be possible for a man to have one eye with normal color vision and one eye with color blindness?
a. the woman is heterozygous, with one X chromosome bearing the allele for normal vision and one X chromosome with the allele for color blindness. one of the two X chromosomes is inactivated at random during early embryogenesis. If one eye derived exclusively from cells that inactivated the normal X, then that eye would be color blind, whereas the other eye may e derived from progenitor cells that inactivated the color blind X
b. one way would be for the man to be XXY and the answer to part a would abbly.
how many barr bodies would you expect to see in human cells containing the following chromosomes?
a. XX-1 barr body
human cells inactivate all X chromosomes beyond one. The Y chromosome has no effect on X inactivation
how do incomplete dominance and codominance differ?
imcomplete dominance means the phenotype of the heterozygote is intermediate to the phenotypes of the homozygotes.
codominance refers to situations in which both alleles are expressed and both phenotypes are manifested simultaneously
what is incomplete penetrance and what causes it?
incomplete penetrance occurs when an individual with a particular genotype does not express the expected phenotype. Environmental factors, as well as the effects of other genes, may alter the phenotypic expression of a particular genotype
what is a complementation test and what is it used for?
complementation tests are used to determine whether different recessive mutations affect the same gene or locus(are allelic) or whether they affect different genes. The two mutations are introduced into the same individual by crossing homozygotes for each of the mutants. If the progeny show a mutant phenotype, then the mutations are allelic(in the same gene). If the progeny show a wild type(dominant) phenotype, then the mutations are in different genes and are said to complement each other because each of the mutant parents can supply a functional copy(or dominant allele) of the gene mutated in the other parent
what is genomic imprinting?
genomic imprinting refers to different expression of a gene depending on whether it was inherited from the male parent or the female parent
what is the difference between a sex influenced gene and a gene that exhibits genomic imprinting
for a sex influence gene, the phenotype is influenced by the sex of the individual bearing the genotype.
For an imprinted gene, the phenotype is influenced by the sex of the parent from which each allele was inherited
what are continuous characteristics and how do they arise?
continuous characteristics exhibit many phenotypes with a continuous distribution. They result from the interaction of multiple genes(polygenic traits), the influence of environmental factors on the phenotype, or both
when a chinese hamster with white spots is crossed with another hamster that has no spots, 1/2 of the offspring have white spots and 1/2 have no spots. When two hamsters with white spots are crossed, 2/3 of the offspring possess white spots and 1/3 have no spots
a. what is the genetic basis of white spotting in chinese hamsters?
b. how might you go about producing chinese hamsters that breed true for white spotting?
the 2:1 ratio when two spotted hamsters are mated suggests lethality and the 1:1 ratio when spotted hamsters are matted to hamsters without spots indicated that spotted is a heterozygous phenotype. Using S and s to symbolize the locus responsible for white spotting, spotted hamsters are Ss and solid colored hamsters are ss. One quarter of the progeny expected from a mating of two spotted hamsters is SS, embryonic lethal, and missing from those progeny, resulting in the 2:1 ratio of spotted to solid progeny
b. because spotting is a heterozygous phenotype, it should not be possible to obtain chinese hamsters that breed true for spotting, unless the locus that produces spotting can be separated form the lethality
a dog breeder liked yellow and brown labrador retrievers. In an attempt to produce yellow and brown puppies, he bought a yellow Lab male and a brown Lab female and mated them. All the puppies produced in this cross were black
a. explain the result
b. how might the breeder go about producing yellow and brown Labs?
a. Labs vary in two loci, B and E. Black dogs have dominant alleles at both loci(B_E_), brown dogs have bbE_ and yellow dogs have B_ee or bbee. because all the puppies were black, they must all have inherited a dominant B allele from the yellow parent and a dominant E allele from the brown parent. The brown female parent must have been bbEE and the yellow male must have been BBee. The black puppies were all BbEe
b. mating yellow with yellow will produce all yellow Lab puppies. Mating two brown Labs will produce either all brown puppies, if at least one of the parents is homozygous EE, or 3/4 brown and 1/4 yellow if both parents are heterozygous Ee
in some goats, the presence of horns is produced by an autosomal gene that is dominant in males and recessive in females. A horned female is crossed with a hornless male. The F1 offspring are intercrossed to produce the F2. what proportion of the F2 females will have horns
H+=allele for the presence of horms
H-=allele for hornlessness
Since H+ is recessive in females, the horned female parents must be H+H+. The hornless male is H-H- because the absence of horns is recessive in males. Then their F1 progeny must be all heterozygous H+H-. An intercross of the F1 would produce both male and female progeny in the ration of 1H+H+, 2 H+H-, and 1 H-H-. Again remembering that H+ is recessive in females, we would expect a ratio of 3:1 hornless to horned females.
shell coiling of the snail Limnaea peregra results from a genetic maternal effect. An autosomal allele for a right handed shell (s+) called dextral, is dominant over the allele for the left handed shell (s) called sinistral. A pet snail called Martha is sinistral and reproduces only as a female.
True or false...
a. martha's genotype must be ss
b. martha's genotype cannot be s+s+
c. all the offspring produced by martha must be sinistral
d. at least some of the offspring produced by Martha must be sinistral
e. Martha's mother must have been sinistral
f. all Martha's brothers must be sinistral
a. false. for maternal effect genes, the phenotype of the individual is determined solely by the genotype of the individual's mother. So we know Martha's mother must have been ss because Martha is sinistral. If Martha was produced as a result of self fertilization, then martha must indeed by ss. But if Martha was produced by cross fertilization, then we cannot know Martha's genotype without more information
b. True. martha's mother is ss, so Martha must be s+s or ss
c. false. because we dont know martha's genotype, we cannot predict the phenotype of her offspring
d. false. if martha is s+s then all her children will be dextral, if martha is ss then all her children will be sinistral
e. false. martha's mother's phenotype is determined by the genotype of her mother. We know Martha's mother's genotype must have been ss so her mother's mother had at least one s allele.
f, true. because Martha's mother must have been ss, all her progeny must be sinistral
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