Mendelian Genetics

Laws of Inheritance

Terms

Phenotype: • the appearance of the trait in the organismGenotype: • the genetic make-up of an organismGenetics: • biology dealing with the principles of variation and

inheritance in animals and plants. • gives us greater understanding of how we can determine

the likelihood of inheriting certain phenotypes.

History• Since 460 B.C.E philosophers, physicians and scientists have

been theorizing heredity

• Most believed inheritance involved a “mixing” of genetic information from the mother and father

• During the 19th century the “blending theory” of genetics was quite popular.

-offspring’s phenotypes were a blend of the parents’

• Didn’t understand how phenotypes were expressed, or WHY some phenotypes in the P generation were NOT expressed in their offspring’s generation

Gregor Mendel (1822-1884)

• Austrian monk

• Developed principles that became genetics through 8 yrs of conducting experiments on pea plants

Why the pea plant?• Commercially available• Easy to grow, and matures quickly• Sexual organs (re: reproductive organs) entirely enclosed in

the flower – meaning plants self-pollinate so Mendel could control reproduction

Mendel’s Experiments• introduced pollen from the flower of one pea plant to the

pistil (ovaries) of another flower• resulted in a variety of different, easily distinguishable

phenotypes• examined seven of the resulting phenotypes – each of which

only had two possible variations

Experiment #1 – F1 Generation• obtained plant “purebreds” (P* – parent generation) *only use this notation if purebred

• crossed each P generation with its opposite-ex: tall P plant is crossed with a short P

• this produced the F1* generation (1st filial) -hybrids since a cross between two different purebreds *only use this notation if hybrid offspring of 2 purebreds

• called a Monohybrid Cross (only 1 trait was selected – plant height

Results:• all F1 plants were tall• concluded the trait for tall plants must be dominant • trait for short plants must be recessive

Dominant:• a phenotype that is always expressed

Recessive: • a characteristic that is always latent (present but inactive) • not usually expressed in an individual

Principle of Dominance: • When individuals with contrasting traits are crossed, the offspring

will express only the dominant trait

F1 Results

Experiment #2 – F2 Generation• Crossed F1 generations• produced F2 generationResults:• 3 were tall, 1 was short• ratio of 3:1 is known as the Mendelian ratio

Mendel’s Law of Segregation1. Each parent has 2 copies of each gene

Gene – part of the chromosome that controls expression of a certain trait

2. Genes can exist in different forms, known as alleles.• Alleles for pea seed shape include round and wrinkled.

3. Offspring inherit one allele of a gene from each parent.• If both alleles are the same = homozygous for that gene.• If alleles are different = heterozygous for that gene.

4. Alleles can be dominant or recessive.• Dominant alleles are expressed in heterozygous offspring.• Recessive alleles are only expressed in homozygous offspring.

5. Each gamete carries only one allele for each trait.

Genotypes & Phenotypes• Capital letters used for dominant allele• lowercase of the same letter used for recessive• ex. round seeds are dominant over wrinkled.• R = round, r = wrinkledGenotype • the alleles for an individual, ex. for seeds:

- RR (homozygous round)- rr (homozygous wrinkled)- Rr (heterozygous round)

Phenotype• the expressed trait for an individual (what is seen) ex. for seeds:

- round- wrinkled

• Heterozygous genotypes (Rr) always have the phenotype of the dominant allele.

Plant Height Experiments• F1 genotype:- 4 Tt (heterozygous)• F1 phenotype:- 4 tall• F2 genotypes:- 1 TT (homozygous dominant)- 2 Tt (heterozygous)- 1 tt (homozygous recessive)- ratio 1:2:1• F2 phenotypes:- 3 tall- 1 short- ratio 3:1

tt

P Generation

F1 Generation

F2 Generation

True-breeding parents

Tall = dominant = TShort = recessive = t

TT tt

Hybrid offspringAll heterozygous

all Tt

Monohybrid crossfrom F1 generation

TT Tt Tt

Tall

all Tall

TallTall Tall

short

short

Genotypes1 TT : 2 Tt : 1 tt

Phenotypes3 Tall :1 short

Probability & Punnet SquaresProbability – the likelihood of an outcomePunnett square - used to determine the probabilities of allele combinations when the genotypes of the parents are known

Making Punnet Squares

Practice Problem #1• A geneticist crosses two pea plants. One of the plants is heterozygous

for the dominant inflated pea pod trait, and the other plant has constricted pea pods. What would be the expected genotypic and phenotypic proportions of the offspring?

Given• inflated pod (I) - dominant • constricted pod (i) - recessive• II or Ii = Inflated pods• ii = constricted pods• heterozygous inflated x constricted = Ii x iiRequired• Punnet square• gametes from the cross• expected proportions of F1 plants with genotypes II, Ii, and ii• expected proportions of phenotypes (inflated pods and constricted)

Solution• The Ii parent produces

gametes I and i.• The ii parent produces

gametes i and i.

I i

i Ii ii

i Ii ii

Paraphrase1/2 of the offspring would be heterozygous (Ii) and 1/2 would be ii. Therefore, 1/2 of the offspring would have inflated pods, and 1/2 would have constricted pods.

Solutions #2- 42. Solution• The Aa parent produces gametes A and a.• The aa parent produces gametes a and a. Paraphrase• ½ the offspring would be heterozygous (Aa) and ½ would be

aa. Therefore, ½ of the offspring would be brown, and ½ would be albino

3. Solution• The DD parent produces gametes D and D.• The Dd parent produces gametes D and d. Paraphrase• ½ of the offspring would be heterozygous (Aa) and ½ would

be homozygous (AA). Therefore, all of the offspring would have dimples.

4. Solution• The Tt parents produce gametes T and t. Paraphrase• ½ of the offspring would be heterozygous (Tt), ¼ would be

homozygous dominant (TT) and ¼ would be homozygous recessive (tt). Therefore, ¾ of the offspring would taste PTC, and ¼ would not.

A aa Aa aaa Aa aa

D DD DD DDd Dd Dd

T t

T TT Tt

t Tt tt

Test Crosses• how genotype can be determined from phenotype

• breed an individual with the dominant phenotype with a homozygous recessive

• If offspring are all dominant phenotype, the parent was homozygous dominant

• If offspring are half dominant phenotype, and half recessive phenotype, the parent was heterozygous dominant.