Common Sources of Variation
- independent assortment of chromosomes
- crossing over
- random fertilization
The exhcange of corresponding segments between two homologous chromosomes that occurs during prophase I of meiosis
The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote
After meiosis I or II, results in cells with extra chromosomes (N-1 or N+1)
Trisomy and monosomy are examples: an individual's cells have too many or too few copies of a chromosome
many flowering plant species are this: they have three or more complete sets of chromosomes (3n or 4n)
New species that have more than two sets of homologous chromosomes in each somatic cell
Everytime the plant cell is fertilized, offspring always have the same characteristics. The end result is guaranteed.
Two different true breeders (like one white and one purple) don't produce a guaranteed outcome.
mating of two organisms that differ in only one character.
P Generation --> F1 generation --> F2 generation
Four Hypotheses Developed from Monohybrid Cross
1. There are alternative forms of genes called alleles
2. For each inherited characteristic, an organism has one gene from each parent (2 genes total)
3. Alleles can be dominant or recessive
4. Gametes carry only one allele for each inherited characteristic
Law of Segregation
Gametes have alleles that are independent of one another
- have genes at specific loci
- have alleles of a gene at the same locus; one may have the dominant gene and one the recessive
Cross of two true-breeding traits (AABB x aabb)
Can determine an unknown genotype: is the individual heterozygous or homozygous dominant. Individual is crossed with a homozygous recessive to figure out the unknown genotype
Law of Independent Assortment
During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene paris
Shows the history of a trait in a family and allows researchers to analyze human traits and predict the likelihood of a disorder being passed on
On a sex chromosome (not autosome)
Autosomal Recessive Inheritance
Autosomal recessive alleles are expressed only in homozygotes. Heterozygotes are carriers and do not have the trait.
50% chance the child will be a carrier and 25% chance of the child expressing the trait
Will be passed along to offspring, and 50% chance the disorder will be expressed. Carriers who live to reproducing age are heterozygous Aa. More likely to result in early death/affected
Incomplete Dominance in Humans
Hypercholesterolemia: a human trait that is incompletely dominant, leads to inability to make LDL receptors
Two nonidentical alleles of a gene are both fully expressed in heterozygotes so neither is dominant or recessive. This can occur in multiple allele systems (ABO blood type)
This type of gene influences multiple traits.
- Marfan syndrome
Two or more gene products influence a trait, and typically one gene product suppresses the effect of another
- Alleles B and b designate dog color, and two recessive alleles ee suppress the color
Environment and Gene Expression
genotype + environment --> phenotype
Traits within a range of small differences; the more factors that influence the trait, the more continuous the distribution of the phenotype. Forms a bell curve when graphed.
- shades of skin
Discovered in fruit flies first, when researchers noticed males tended to have white eye color and females had red. This white color was a recessive trait located on the X chromosome, so males were more likely to get it.
Sex-Linked Recessive Disorders
- red/green color blindness
- duchenne muscular dystrophy
- hemophilia A
Embryo/fetus is tested before birth to screen for sex/genetic problems.
- noninvasive: obstetric sonography
- fetoscopy (camera inserted into womb)
- amniocentesis (fluid sample of sac for karyotyping)
- chorionic villus sampling (direct tissue sample)
Problem with Amniocentesis
You get cells from both mom and baby