Genetics Fall 2015 Lecture 9
Terms in this set (45)
What are sex chromosomes?
chromosomes whose presence or absence functions in the determination of the sex of the organism
Sex chromosomes were first observed, but not understood, by __________ in 1891 during investigations of spermatogenesis in insects.
Honking identified a particular structure in the nucleus that was present in 1/2 the sperm and _______ in the other 1/2, which he then termed the "__ body".
absent; the X body
In 1902, McClung observed the same thing as Henking in grasshoppers and associated the "X body" with sex determination, but incorrectly associated it's presence with _____________.
Wilson worked out the problem Henking and McClung couldn't quite get in the _____ _______ bug.
In the Box Elder bug that Wilson was using, he discovered males gamete contained either __ or ___ chromosomes, while female gametes always contained ___ chromosomes.
6 or 7; 7
Wilson's system: what did he conclude?
Wilson's system: the "X body" was the ____ chromosome in 1/2 the male gametes.
Wilson's system: he called the chromosome that determined the sex the ___ chromosome after Henking.
Simple Sex Determination: if two X chromosomes are present the individual develops into a __________.
If only one X chromosome is present the individual develops in a _____.
In the simplest form of sex determination, males will have an ____ number of chromosomes
Do humans use the simplest form of sex determination?
Wilson also observed the Y chromosome in the ___________ bug.
In his milkweed bug observations Wilson discovered each sex had the _____ number of chromosomes.
Females had 2 like-sized
Males have 1 large X similar to females, but have 1 small __, which was not homologous with the entire X chromosome
What is a homogametic pair?
2 of the same sex chromosome
What is a heterogametic pair?
2 different sex chromosomes
In the case with the milkweed bug and the Box Elder Bug the _____ is the sex determining parent.
male [male produces gametes with either an X or a Y/none]
Mendel's Factors and Sex Determination
Early evidence that Mendel's factors (the genes) actually reside on chromosomes due to the association of an obvious phenotype with a particular chromosomes; further supports the parallels between Mendel's factors and chromosome behavior
homogametic female XX; heterogametic male with just one X; female will be 2n and even; male will be 1n and odd; male is sex determining parent; presence of second X = becomes a female; characteristic of a lot of insect groups
homogametic female XX; heterogametic male XY; Y chromosome is morphologically different from the X (Y is very small and contains a lot of heterochromatin and X is very large); male is sex determine parent; Y is sex determining factor; very common is higher level organisms such as mammals and humans
Homogametic sex means
producing only one kind of gamete (female in the XX systems)
Heterogametic sex means
producing two (or more) types of gametes [male in the XY and XO systems]
female is heterogametic ZW; male is homogametic ZZ; W is morphologically different from the Z [Z is very large and W is very small and contains a lot of heterochromatin]; female is sex determining parent; W is sex determining factor; common in birds and butterflies
Multiple Sex Chromosome system X1X1X2X2X1X2Y
several variations possible; there may be more than one pair of chromosomes involved in sex determination; no sex determining parent; occurs in beetles and aphids
Neo XY system
XO -> X1X2neoY
very complex event; one of the few times where we can actually catch chromosomes changing in the process and its like watching evolution in action; involves a reciprocal translocation between the X chromosome and an autosome; has a developing Y chromosome; movement of material; occurs in insects
Balanced Sex Determination system
female determiners reside on the X; male determiners reside on the autosomes; all individuals carry the genes for both sexes; ratio determines sex; altering of ratio can produce matamale or metafemale; metafemale: exagerrated; tetraploid: look normal but don't do well; intersex: both male and female parts; More Xs=more female; more Ys=more male; occurs in Drosophila and other flies
The balanced sex determination system was observed by
Bridges in Drosophila
In the case of tetraploidy in the balanced sex determination system, sex is not due to the polyploidy but due to the ____ ratio.
normal (4/4 = female and 2/4 = male)
Haplodiploid Sex Determination system
2n = female; 1n = male; diploid female can either lay fertilized (2n) or unfertilized (1n) eggs; both develop into functional organisms but different sexes; homozygous individuals become weak sterile males that die off; hemizygous individuals become male; heterozygous individuals become female; works through a multiple allele series; females are sex determining parent; females make decision on sex of offspring on a case by case basis; 9 different alleles present at the same loci; occurs in social insects such as wasps, ants, etc.
The 9 different alleles present at the same loci in the haplodiploid sex determination system each occur in about ____ numbers (___%).
in about equal numbers; 12%
Homozygous individuals in the haplodiploid sex determination system occur in 1 in every ___ births.
Hemizygous refers to
a gene which is present in a single dose or a single copy rather than the normal two copies in a diploid
Hemizygosity can be due to
the loss of genetic material, heterogametic sex, haploid state, gametes, etc.
Genic Sex Determination system
sex is determined by alleles present at one or more genes; monecious: both sexes present; diecious: one sex present; allele's presence at one or more genes determines sex; no sex determining parent; corn in an example
Genic Based System of Corn
Bs __ Ts __ normal plant
bs bs Ts __ male plant
Bs __ ts ts female plant
bs bs ts ts female plant
simple dominance here; normal are monecious
Environmental Sex Determination System (nongenetic)
male and female with similar genotype; stimulus from the environment determines sex (ex. temperature for turtles); factors are in the environment; occurs in turtles and Bunnelia
Environmental Sex Determination example of temperature for turtles:
Eggs incubated at 25 degrees C = all male
Eggs incubated at 30 degrees C = all female
Eggs incubated at 27.5 degrees C = about 50/50 female/male
Turtles have a really low
The 3 temperature dependent types: case 1, case 2, and case 3
case 1: low temperature = all female, high temperature = all male
case 2: low temperature = all male, high temperature = all female
case 3: high or low temperature = all female, intermediate = all male
What is aromatase?
the key enzyme in sex determination using temperature as the determining factor
What does aromatase do?
converts androgens (male sex hormone) into estrogens (female sex hormone)
Aromatase: there is a temperature dependent switch based on
production and activity of the enzyme.
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