Introduction to

Genetics

We’ve all heard of it, but … What is genetics?

Genetics: the study of gene structure and

action and the patterns of inheritance of traits from parent to offspring.

Ancient ideas about genetics

•Selective breeding of plants and animals •Examples:

Ancient ideas about genetics

•Theories why offspring resemble parents •Spontaneous generation?

•Miniature pre-formed organisms?

•Blending of “vital fluids”?

Then, in 1860…

Gregor Mendel successfully discovered genetic principles

Pea Characteristics Mendel studied traits of pea plants one at a time

Mendel and Plant Breeding

• Mendel examined thousands of crosses and offspring

• Mendel analyzed

his results mathematically- saw patterns no one else saw

Mendel’s

experiments

Mendel’s Principle Findings

•Dominant and Recessive

•Certain characteristics are dominant to others

•The dominant trait will mask another (recessive) trait, preventing it’s expression

•The recessive trait “disappears” for a generation

Hybrid in this sense is a word for a having a genotype that is heterozygous

True-breeding means the organism has a genotype that is homozygous.

•Law of Segregation •For each trait, organisms have 2 genes

• (which may be different versions or forms of the genes, these are called alleles)

•Each pea plant got one gene from each parent

•Genes are segregated from each other during egg or sperm formation (meiosis).

**Draw in chromosomes for a 2n cell of 4.

(Use 2 different colors)

Mendel’s Principle Findings

Remember Meiosis?!? The Law of Segregation is the SAME THING! Mendel just theorized about it before it was actually discovered.

diploid (2n)

Segregation of Genes during Meiosis

meiosis I

sperm

haploid (n)

meiosis II

Bb

B

b

B

B

b

b

Mendel’s Principle Findings

•Independent Assortment •Factors for different traits are distributed to reproductive cells independently

•Later it was seen that orientation of homologous pair to poles during meiosis is random. •So, for example, genes from the individual’s mother

don't all stay together Click here for an animation!

Some terms …

•Trait: characteristics of an

organism passed from generation to generation. •Eye color

•Skin color

•Hair color and type

•Height

•Temperament

•Symptoms for genetic diseases

Not all traits are easy to predict… For example: polygenic inheritance. (Where have we seen the prefix “poly” before?)

Genotype

•Tells you the genetic makeup of the organism

•The genes the organism carries

Genotype •Written using symbols

•Each allele is represented by a letter

•Capital represents dominant trait

•Tall = T

•Lower case represents recessive trait

•Short = t

•Capital letter is always written first.

Homozygous •An individual which contains one allele for a genetic trait

•TT-- homozygous dominant

•tt -- homozygous recessive

Heterozygous •An individual which contains different alleles for a genetic trait

•Tt -- heterozygote

Phenotype

•What the organism looks like (THE TRAIT THAT IS EXPRESSED)

•Controlled by the genotype

•TT---------- tall

•Tt----------- tall

•tt------------ short TT Tt tt

Remember:

Example

eye color gene

Allele b

(blue eyes)

eye color gene

Allele B

(brown eyes)

Paternal Maternal

This person would have brown eyes (Bb)

Phenotype

Genotype

Punnett squares • The probability the next generation will exhibit a certain

trait can be shown with a Punnett square.

• The genotype of the mother is shown on the top of the square (TT) • The genotype of the father is shown on the side of the square (tt) • The various combinations of genes (genotypes) are shown in the squares. All of these offspring will be (T) tall and carry the recessive (t) short allele.

T T

t

t

Tt

Tt Tt

Tt

Monohybrid Cross (crosses that examine the inheritance of only ONE specific trait)

•What happens if you cross a tall (heterozygous) plant (Tt) with a short plant (tt)?

Genotypic Ratio:

TT : Tt : tt

0 : 2 : 2

Phenotypic Ratio:

Tall : Short

2 : 2

T t

t

t

Tt

tt Tt

tt

Punnett squares •If deafness in dogs is recessive (dd), show what offspring could result from two dogs that are heterozygous (Dd)

3 possible genotypes: DD, Dd, dd Genotypic Ratio: DD : Dd : dd 1 : 2 : 1 2 possible phenotypes: Not deaf or Deaf Phenotypic Ratio: Not Deaf : Deaf 3 : 1

D d

D

d

DD

dd Dd

Dd

Test Cross

• Farmer Dan breeds guinea pigs.

• There is a gene in guinea pigs for teeth length. Teeth can either be normal (T) or bucked (t).

• Farmer Dan buys what he thinks is a normal TT guinea pig. How can he be sure it is “pure” (TT) and not a heterozygote (Tt)?

Test Cross: Breed the unknown genotype with a homozygous recessive.

A homozygous dominant guinea pig will result in offspring that all have normal teeth.

T T

t Tt Tt

t Tt Tt

T t

t Tt tt

t Tt tt

A heterozygous guinea pig will result in a 2:2 or 1:1 ratio of normal to buck-teethed offspring.