AP Biology Genetics Column #1

Laws of Probability

* A probability of zero means that the event cannot happen * A probability of one means that the event must happen. * All probabilities must be between zero and one inclusively. * The sum of probabilities of all simple events must be equal to one

trihybrid cross

3 pairs of genes that sort independently F2 generation phenotypic ratio 27:9:9:9:3:3:3:1 each with probability of 1/64

Barr body

A dense object lying along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome.

test cross

A genetic cross in which a test organism showing the dominant trait is crossed with one showing the recessive trait; used to determine whether the test organism is homozygous dominant or heterozygous.

gene

A segment of DNA on a chromosome that codes for a specific trait

epistasis

A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.

sex linked

An inheritance pattern in which traits are controlled by genes located on the X chromosome.

Mendel's Law

Law of Independent Assortment - pairs of alleles for different genes segregate independently of each other.

P generation

The parent individuals from which offspring are derived in studies of inheritance; P stands for parental.

Punnett Square

a chart that shows all the possible combinations of alleles that can result from a genetic cross

dihybrid cross

a cross between individuals that have different alleles for the same gene

sex influenced traits

an autosomal trait that is influenced by the presence of male or female sex hormones

autosome

any chromosome other than a sex chromosome

dominant

describes a trait that covers over, or dominates, another form of that trait

monohybrid cross

hybridization using a single trait with two alleles (as in Mendel's experiments with garden peas)

incomplete dominance

one allele is not completely dominant over the other allele

allele

one of the alternative forms of a gene that governs a characteristic, such as hair color

sex chromosome

one of two chromosomes of the 23rd pair of human chromosomes, which determine an individual's gender

phenotype

physical characteristics of an organism

heterozygous

term used to refer to an organism that has two different alleles for the same trait

homozygous

term used to refer to an organism that has two identical alleles for a particular trait

F1 generation

the first offspring from a cross of two varieties in the parental (P) generation

genotype

the genetic makeup of an organism

F2 generation

the second generation of offspring, obtained from an experimental cross of two organisms; the offspring of the F1 generation

X chromosome

the sex chromosome found in both men and women. females have two x chromosomes; males have one. an x chromosome from each parent produces a female child

Y chromosome

the sex chromosome found only in males. When paired with an X chromosome from the mother, it produces a male child.

locus

the specific site of a particular gene on its chromosome

multiple alleles

three or more forms of a gene that code for a single trait

recessive

trait that will only appear in the phenotype if organism inherits two of them; covered up by the dominant gene

codomiance

where both alleles contribute to the phenotype of the organism (roan cows)