Ch 8: Mendel- Monohybrid and Dihybrid cross

lecture slides and notes

Inheritance

traits are passed from parents to offspring

Blending inheritance

• hypothesis of mid-1800’s

• offspring inherit characteristics that are a blend that are a blend of those seen in the parents

Particulate inheritance

• determinates = genes

• determinates are passed to the next generation without blending

Gregor Mendel

• father of genetics

• performed genetic crosses with pea plants

• discovered two laws of inheritance

What did Mendel do to his pea plants?

• controlled mating of pea plants by removing pollen from stamens and manually placing it on pistils of..

  • another plant = cross fertilization

  • same plant = self-fertilization

Why did Mendel do what he did to pea plants?

• he wanted to understand how traits in pea plant are passed on from parents to offspring (inherited)

Characters vs. traits

• Characters: observable physical features

  • flower color, seed color

• Traits: forms of a character

  • purple vs. white flowers

  • round vs. wrinkled seeds

True breeding

• when self-fertilized or crossed with one another, they produce offspring with the same trait(s) as the parents

• Mendel selectively bred plants to create

  • true breeding purple-flowers

  • true-breeding white flowers

Reciprocally crossed

• Mendel reciprocally crossed true breeding plants (purple and white)

• Reciprocal cross: pollen is transferred between plants, with each one giving and receiving

  • Parent (P) generation: first set of plants crossed

  • F₁generation: offspring of P generation

  • F₁ generation were monohybrids: offspring of two individuals that differ in a single trait

Monohybrid

mating of two true-breeding lines

F₂ generation

• Mendel allowed the F₁ generation to self-pollinate to produce F₂ (second filial) generation

• this was his monohybrid cross

A pattern from Mendel’s experiment emerged

• one trait disappeared in the F₁ generation and reappeared in the F₂

• the F₂ traits were always in a 3:1 ratio

  • ¼ had the trait that disappeared in the F₁

• this disproved blending inheritance

F₃ generation

• Mendel found an underlying pattern when considering potential offspring

• underlying pattern = 1:2:1 ratio

What did Mendel decide determine traits?

pairs of discrete particulate genes

Genes

a sequence of DNA at a specific locus that performs a function

Alleles

different versions of the same gene

• example: A = purple flower, a = white flower

Genotype

allelic makeup of an individual

• example: Aa

Phenotype

outward expression of the genotype

• example: purple flower

Dominant trait

requires only one copy of that allele to be expressed (A)

Recessive trait

requires both copies of that allele to be present (a)

Homozygous genotype

two alleles at a locus are the same (AA or aa)

Heterozygous genotype

two alleles at a locus are different (Aa)

Laws of Inheritance

1. Law of Segregation

2. Law of Independent Assortment

Law of Segregation

1. during the formation of gametes, alleles separates equally so that each gamete receives only one copy

   1. Meiosis = process of create gametes

   2. 3:1 phenotypic ratio and 1:2:1 genotypic rati

Punnett Square

• write alleles that can be given (one allele per gamete) by one parent along the top and those from the other parent on the left

• combine alleles from the top and side to create the possible genotype combinations for the offspring

  • intersections of columns and rows represent fertilization between gametes

Interpreting Punnett squares

1. Character: flower color

   1. A = purple flower = dominant

   2. a = white flower = recessive

2. monohybrid cross: Aa x Aa

   1. both heterozygotes

3. offspring genotypic ratio: 1:2:1 (AA, Aa, Aa, aa)

4. offspring phenotypic ratio: 3:1 (3 purple, 1 white)

Test Cross

• plant with dominant phenotype but unknown genotype (A_) is crossed with a plant with a recessive homozygous genotype (aa)

• resulting offspring were either..

  • 100% dominant phenotype

  • 50% dominant phenotype and 50% recessive phenotype

• pattern aligns with law of segregation

Overview of Monohybrid cross

Overview of F₁ generation cross

What do Punnett squares show us?

probability of each combination of alleles —> predicted ratio is statistically likely, but not guaranteed

Alleles are segregated ____ and _____ into _____

Alleles are segregated equally and randomly into gametes

Passing on an allele is like a coin flip…

but some people have a double sided coin

Rules of Probability

1. Event is certain to happen, then probability = 1

   1. like a double-headed penny landing on heads

2. Event cannot possibly happen, probability = 0

   1. like a double-headed penny landing on tails

3. all other events with multiple possible outcomes

   1. probability between 0 and 1

   2. standard penny flip

What do the rules of probability allow us to do?

predict possible phenotypic outcomes

Multiplication rule

multiply the probability of independent events to find the probability of both happening together

• probability of two people flipping a coin and both getting heads = ½ x ½ = ¼

Addition rule

add the probabilities of mutually exclusive events to find the probability of either event happening

• probability of either getting tails-heads or heads-tails = ¼ + ¼ = ½ chance of either

Mendel’s use of the dihybrid cross

used to observe the inheritance pattern of pairs of traits

Mendel’s observations in his dihybrid cross

1. F₁ generation: two traits temp disappeared

2. F₂ generation: any combination of traits was possible; always in a 9:3:3:1 phenotypic ratio

Law of Independent Assortment

alleles of different genes are distributed into gametes independently of each

• assortment of the seed shape and seed color alleles are independent events (do NOT impact each other)

What can you predict using dihybrid crosses?

find the predicted offspring ratio of a dihybrid cross using a Punnett square

Law of segregation says…

each gamete gets one copy of each allele

Law of independent assortment says…

alleles of different genes are distributed into gametes independently of each other

Ratios of a dihybrid Punnett square

1. Phenotypic ratio: 9:3:3:1

2. Genotypic ratio: 1:2:2:1:4:1:2:2:1

What is a predigree?

diagram showing inheritance pattern of a trait within a family —> allows us to trace the inheritance of rare alleles