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Common Sources of Variation

- independent assortment of chromosomes
- crossing over
- random fertilization
- nondisjunction
- mutations

Crossing Over

The exhcange of corresponding segments between two homologous chromosomes that occurs during prophase I of meiosis

Random Fertilization

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

True-Breeding Plants

Everytime the plant cell is fertilized, offspring always have the same characteristics. The end result is guaranteed.

Hybrid Plants

Two different true breeders (like one white and one purple) don't produce a guaranteed outcome.

Monohybrid Cross

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

Homologous Chromosomes

- have genes at specific loci
- have alleles of a gene at the same locus; one may have the dominant gene and one the recessive

Dihybrid Cross

Cross of two true-breeding traits (AABB x aabb)

Test Cross

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

Family Pedigree

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

Sex-Linked Disorder

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.
- albinos
- tay-sachs
50% chance the child will be a carrier and 25% chance of the child expressing the trait

Dominant Disorders

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

Continuous Variation

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

Sex-Linked Genes

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

Prenatal Diagnosis

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

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