Probability & Genetics

.A. Learning goals

Explain the random process of chromosome segregation and distribution of alleles in gametes.

Predict possible combinations of alleles in a zygote from the genetic makeup of the parents.

B. Independent Assortment of Alleles

Gametes (sperm and egg) will contain two alleles for each gene. They are divided randomly during meiosis.

Sperm + Egg Yy yy

Yy Yy yy yy *this random selection of alleles occurs for23 pairs of chromosomes for thousands of genes !!!

Probability

C. Punnett Square Terminology Homozygous: Individuals with identical alleles Ex.

RR, rr Heterozygous: Individuals with different alleles ex.

Rr Dominant: always expresses the trait Rr, RR Recessive: needs both recessive alleles to express the

trait rr Phenotype – physical characteristic Ex. White fur, blue eyes, green skin Genotype – genetic makeup Ex. FF, Bb, or gg

How to use Punnett Squares

Choose a letter to represent the alleles in the cross.

Write the genotypes of the parents. Determine the possible gametes

(reproductive cells) that the parent can produce.

Enter the possible gamete at the top and side of the Punnett square.

D. Practice Ex. 1

A plant that is homozygous recessive for short , _tt___, is crossed with a plant that is homozygous for TT. How many offspring will be short?

T T

t

t

Tt Tt

Tt Tt

Answer: O , all will be tall and heterozygous.

Ex. 2

The Rock has the heterozygous condition of rolling tongue, Rr and his wife has the heterozygous condition as well, Rr. They have four children. What is the ratio of rolling tongue among the offspringT T

R

r

RR Rr

Rr rr

R r Answer: 3:1 of the offspring have rolling tongue!

Punnett Squares Monohybrid & Dihybrid Crosses

Punnett Squares

Mendel’s Principles Summary

The inheritance of biological characteristics are determined by genes.

For two or more forms of a gene, dominance and recessive forms may exist.

Most sexually reproductive organisms have two sets of genes that separate during gamete formation.

Alleles segregate independently.