GeneticsA study of inheritance

Gregor Mendel

• Father of modern genetics

Gregor Mendel

• Father of modern genetics

• Researched with pea plants

Gregor Mendel

• Father of modern genetics

• Researched with pea plants

• Developed ideas of dominance and trait segregation

Phenotype

• Phenotype – Physical

characteristics

Genotype

• Phenotype – Physical

characteristics

• Genotype – Genes we inherit

from our parents

Phenotype

– Facial structure

Notice the similarities:

Phenotype

– Facial structure– Eyes

Notice the similarities:

Phenotype

– Facial structure– Eyes– Smile

Notice the similarities:

Phenotype

– Facial structure– Eyes– Smile– Ears

Notice the similarities:

Phenotype

– Facial structure– Eyes– Smile– Ears– Nose

Notice the similarities:

Phenotype

– Facial structure– Eyes– Smile– Ears– Nose– Neck

Notice the similarities:

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN

Mendelian GeneticsWe will work with the following three:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN•Flower position - terminal or AXIAL

Mendel's Four Hypotheses• 1) There are alternative forms of “heritable

factors” (what we now call genes).– Alternative forms are called alleles. In our flowers we

have two alleles for color, purple and white.

• 2) Each individual inherits two copies of these alleles. These may be two of the same (e.g., two alleles for purple), or two different ones (one for white, one for purple).– - if an individual has two of the same alleles, it is

termed “homozygous”– - if an individual has two different alleles, it is

“heterozygous”.

Mendel's Four Hypotheses• 3) If the two alleles are different (the

individual is a heterozygote), then one of– the alleles will determine the individual's

appearance. This is the "dominant“ allele.– - the other allele essentially has no effect on the

individual, and is called “recessive”.• 4) a gamete carries only 1 allele for each

trait (because alleles separate from each other during production of gametes.

Homozygous

• Homozygous–Homo means "same or common".

– Homozygous means that both alleles are the same.  Genotypes such as BB, bb, B1B1 are homozygous. 

Heterozygous•   Heterozygous

–Hetero  means "different".   Heterozygous means that the two alleles are not the same.  Thus, genotypes such as Bb, are heterozygous

Predicting Inheritance

To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes.

UPPERCASE letters are used to represent dominant genes.

lowercase letters are used to represent recessive genes.

Predicting Inheritance

For example:T = represents the gene for TALL in pea plantst = represents the gene for short in pea plants

So:TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.

Remember there are two genes for every trait!

Predicting Inheritance

For example:T = represents the gene for TALL in pea plantst = represents the gene for short in pea plants

So:TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.

Remember there are two genes for every trait!

Mendels’ Principle of Dominance

Some genes (alleles) are dominant and others are recessive. The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene.

Predicting Inheritance

Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.

TT x tt

T T t t

Predicting Inheritance

Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.

TT x tt

T T t t

Mendels’ “Law” of Segregation

Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait.

Punnett Squares

Tt Tt

Tt Tt

The genes from one parent go here.

The genes from the other parent go here.

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

F1 generation

Interpreting the ResultsThe genotype for all the offspring is Tt.The genotype ratio is:

Tt - 4/4

The phenotype for all the offspring is tall.The phenotype ratio is:

tall - 4/4

Punnett Squares

T t

T ?? ??

t ?? ??

Your Turn!!

Punnett Squares

T t

T TT Tt

t Tt ttF2 generation

Punnett Squares

T t

T TT Tt

t Tt tt

Next, give the genotype and phenotype ratios of the offspring (F2 generation).

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Phenotype ratio: Tall - 3

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Phenotype ratio: Tall - 3, short - 1

Punnett Squares

T t

T TT Tt

t Tt tt

This is a monohybrid cross. We worked with only one trait. The height of the plant.

Punnett Squares

T t

T TT Tt

t Tt tt

This is a monohybrid cross. We worked with only one trait. The height of the plant.

Nondisjunction• Nondisjunction is the failure of

chromosome pairs to separate properly during meiosis or mitosis.

• Examples of Nondisjunction:• A. Down's Syndrome: 47 chromosomes with 3

#21 chromosomes.• B. Triple-X Syndrome: 47 chromosomes caused

by 3 X chromosomes. – Can cause women to be taller

• C. Klinefelter's Syndrome: 47 chromosomes caused by 2 X chromosomes and 1 Y chromosomes. – Men are unable to have children

• D. Turner's Syndrome: 45 chromosomes with 1 X chromosome (caused by the absence of one of the X chromosomes or a Y chromosome).– Women can not have children and may be shorter.