Genetics and Inheritance

• Inheritance is the transmission of genetic information from one generation to the next

• Genetic information is stored in DNA

Mechanisms of Inheritance

• Homologous chromosomes contain the same genes in the same order

• An allele is a different form of the same gene. For every gene, you have a maternal and paternal allele. These alleles may be exactly the same, or they may be different in some way.

Maternal chromosome

Paternal chromosome

-centromere-

C

D

c

d

Mechanisms of Inheritance

• C and D represent genes on homologous chromosomes. Notice that they have identical locations on the chromosomes.

• Big C and little c are different alleles of the same gene

• Big D and little d are different alleles of the same gene

T T t tT t

Homozygous dominant Homozygous recessiveHeterozygous

Allele combinations

• When both maternal and paternal alleles are the same, they are homozygous.

• When they are different, they are heterozygous.

• When an allele is always expressed, it is dominant. (Big letter)

• When an allele has no visible effect, it is recessive. (little letter)

When we observe patterns in phenotypes that are clear and specific, it gives us clues about what is happening at the level of an individual’s genotype (homozygous dominant, heterozygous, homozygous recessive).

When we observe patterns in phenotypes that are clear and specific, it gives us clues about what is happening at the level of an individual’s genotype (homozygous dominant, heterozygous, homozygous recessive).

Genotype and Phenotype

• An individual’s DNA sequence (genes) constitutes their genotype

Genotype DNA

• The visible result of the underlying genotype is one’s phenotype

Phenotype visible appearance of an individual

Genotype Homo- or heterozygous

Phenotype

FF Homozygous dominant

Lots of freckles

Ff Heterozygous Lots of freckles

F_ Homozygous dominant or heterozygous

Lots of freckles

ff Homozygous recessive

Few or no freckles

Genotype and Phenotype

Gregor Mendel

• Monk• Took courses in math, physics and botany• Used pea plants to determine how

parents transmit genes to offspring

• Mendel’s Idea: Every generation a plant inherits two “units” of information about a trait– One from each parent

Mendel Performed Crosses

• Mated one dominant pea plant with one recessive pea plant– Called the P generation

• Offspring the F1 generation

Cross: AA x aa A A

a

a

Aa Aa

Aa Aa

Gametes from one individual

Gametes from the other individual

Individuals are mated

All of the offspring are heterozygous and red

Practicing Genetic Problem Solving

A Punnett square can predict the possible progeny that would result from a cross. In the example below, allele “A” creates a red apple and allele “a” produces a green apple. A homozygous dominant and a homozygous recessive apple are mated.

A a

A

a

AA Aa

Aa aa

Now let’s cross two of the offspring apples, Aa and Aa. How many of their offspring will be green?

This is a monohybrid cross because it involves one pair of alleles.

F2 Phenotypic ratio is 3:1

Cross the F1 Generation

Dihybrid Cross

• Follows two traits

• Seed shape: Round (R) or oval (r)

• Seed color: Yellow (Y) or white (y)

• P generation: RRYY and rryy

• What would the F1 generation be?

• What would the F2 generation be?

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

RY Ry rY ry

RY

Ry

rY

ry

Phenotypic ratio is 9:3:3:1

Dihybrid Cross Example

Human Genetics

• Some traits controlled by a single gene

• You will analyze your phenotype for some traits to determine your genotype– Homozygous dominant?– Homozygous recessive?– Heterozygous?

• We will calculate class averages

• You will practice some Punnett Squares