Chapter 9 Part 1

Patterns of Genetic Inheritance

Gregor Mendel Deduced Laws of Inheritance

A blended model of inheritance existed prior to Mendel

• Before the science of inheritance was fully understood, breeders understood that parents contributed equally to offspring, but believed it was due to blending.

• Ex. When red and white flowers breed, pink flowers result and when red or white would reappear later, it was due to genetic instability.

• Gregor Mendel - an Austrian monk that did extensive pea plant research starting in the 1860s.

Mendel designed his experiments well

• Garden pea– Easy to cultivate– Short generation time– Produce many offspring– Normally self-fertilize, resulting in individuals

that look like the parent (true breeding)

• Mendel’s design– Removed anthers to prevent self-fertilization– Dusted carpel with pollen from a plant with a different

characteristic– Carpel developed into a pod containing peas– Only one characteristic resulted– If those were bred together (F2 generation) both

characteristics can be found again– Mendel chose easy to identify traits

• Flower color, seed color, seed shape

Single-trait crosses reveal units of inheritance and the law of segregation

Mendel’s law of segregation describes how gametes pass on traits

• Ensured true breeding (offspring always looked like parents) and then performed cross-fertilization experiments

• With previous thinking, breeding these two together should result in an intermediate, but that is not what happened.

• P generation – original parents

• F1 generation – first batch of offspring

• F2 generation – offspring of F1 parents (self-fertilization)

• When he crossed tall and short plants, all offspring were tall.

• When he allowed F1 generation to self-fertilize, 75% were tall and 25% were short, so although F1 did not express that trait (short), it was still able to pass it on to future generations.

• The results led him to the conclusion that the parents contained two separate copies of each hereditary factor, one dominant and one recessive.

• Monohybrid crosses – examining only one trait• Law of segregation

– Each individual has two factors for each trait– The factors segregate (separate) during the formation

of the gametes– Each gamete contains only one factor from each pair

of factors– Fertilization gives each new individual two factors for

each trait

The units of inheritance are alleles of genes

• Alleles – alternative forms of a gene

• Gene locus – location of an allele on a chromosome

• Dominant allele – masks expression of the other allele (capital letter)

• Recessive allele – dominant allele masks the expression of this one (lowercase letter)

Dominant does no mean normal or most frequent

Through meiosis, only one allele for each trait is in each gamete

• Homozygous – organism has two identical allele (homozygous dominant or homozygous recessive)

• Heterozygous – two different alleles – dominant allele expressed

• Genotype – alleles the individual has• Phenotype – physical appearance• Linkage group – grouping of alleles on a

chromosome that will be inherited together (comes into play when looking at inheritance of multiple traits)

Two-trait crosses support the law of independent assortment

Mendel’s law of independent assortment describes inheritance at multiple traits

• Dihybrid cross – two traits are examined• Two possible results when crossing tall with

green pods and short with yellow pods– If the dominant alleles always stay together and the

recessive alleles always stay together, then 2 phenotypes will result in F2 plants (tall with green pods and short with yellows pods)

– If the 4 factors segregate, 4 phenotypes will result• Tall with green pods, tall with yellow pods, short with green

pods, short with yellow pods

• Mendel found 4 phenotypes (phenotypic ratio 9:3:3:1)

• Law of independent assortment– Each pair of factors

separates (assorts) independently (without regard to how the others separate)

– All possible combinations of factors can occur in gametes

• If alleles are not linked, traits will follow this law

• What are the possible genotypes of a tall pea plant with green pods?

Mendel’s results are consistent with the rules of probability

• Punnett square – Diagram used to calculate results of a cross– Can be used to determine probability of offspring

inheriting a specific trait

• In humans, freckles is dominant over no freckles. A man with freckles reproduces with a woman with freckles, but their children have no freckles. What chance did each child have for freckles?

Testcrosses support Mendel’s laws and indicate the genotype

One trait test cross• To confirm F1 were

heterozygous, they were crossed with homozygous recessive. Half should express one phenotype, the other half the other phenotype (1:1 phenotypic ratio)

• Test crosses can determine if genotype is heterozygous or homozygous dominant

Two trait testcross• To determine genotype when a

dominant trait is expressed, it is crossed with one that is homozygous recessive for both traits.

• If homozygous dominant for both traits, only the dominant phenotypes will be expressed for both when crossed.

• If heterozygous for one or both traits, recessive phenotypes will be expressed after cross.