Patterns of Inheritance

Inheritance Hypotheses

• Blending Hypothesis – parental contributions combined

• Particulate Hypothesis – parents pass along discrete heritable units

Some Important Vocab

• Allele= an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

• Homozygous = two identical alleles (same)• Heterozygous = two different alleles

Genotype – The two genes that an organism inherits

for a certain trait (Example Tt, TT, or tt)• Phenotype

– Physical characteristics (example: tall and short)

Analyzing Patterns

• Genetic Cross – controlled experiment to determine the inheritance of a trait– P – parental generation– F1 – first generation

– F2 – second generation

General Patterns of Inheritance

• Alternative versions of genes cause variation

• Offspring inherit one copy from each parent

• Dominant alleles are expressed in phenotype

• Alleles for genes separate during meiosis

• Gametes fuse randomly

Mendel’s Laws• Father of modern genetics• Researched with pea plants • Developed ideas of

dominance and trait segregation– Allelic Interactions– Pleiotropy– Epistasis – Environment– Polygenics

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

Law of Segregation

Law of Independent Assortment • Hypothesis: Inheritance of seed

shape is independent of inheritance seed color.

• Medal crossed two genetic traits:– Yellow vs. Green – Round vs. Wrinkled

• Results: Yes, inheritance of seed shape is independent of seed color.

• Conclusion: Chromosomes assort independently during meiosis

Developed ideas of dominance and trait segregation

• Allelic Interactions– Complete dominance– Incomplete dominance– Codominance

• Pleiotropy• Epistasis • Environment• Polygenics

Punnett Squares A square which can be used to show the random combinations of genes which are possible when a

sperm fertilizes an egg.

Tt Tt

Tt Tt

The genes from one parent go here.

The genes from the other parent go here.

Allelic Interactions

• Complete dominance – dominant allele fully expressed

• Incomplete dominance – neither allele fully expressed

• Codominance – both alleles fully expressed

Allelic Interactions: Complete Dominance

– Dominant vs. Recessive Gene– Dominant Gene: A gene that always

expresses itself. It is symbolized by a CAPITAL letter

– Recessive Gene: a gene that expresses itself only when a dominant form of the gene is NOT present. It is symbolized by a lower case letter

Allelic Interactions: Complete Dominance

• Practice Problem: – Let's say that in seals, the gene for the length of the whiskers has two

alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.

a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?

b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?

Allelic Interactions: Incomplete Dominance • Neither allele fully expressed• the phenotype of the heterozygote will

be intermediate between the phenotypes of the two homozygotes

• Example: RR = Red snapdragon flower

Rr = pink snapdragon flower

rr = white snapdragon flower

Practice Problem: Cross a red snapdragon with a pink snapdragon flower. What will the genotype and phenotype % be for the offspring.

Co-Dominance

• If two alleles have a co dominant relationship, in the heterozygote both alleles will be completely expressed.

• Example: blood type

Example: Blood Types

Co-Dominance

What would happen if you crossed a plant with red flowers with a plant with blue flowers?(a) Give the genotypes and phenotypes for the offspring.(b) How many of the plants would have red flowers? _____%(c) How many of the plants would have purple flowers? _____ %(d) How many of the plants would have blue flowers? _____ %

Sex Linked (X-Linked) Traits

• The genes that are located on the chromosomes are called sex-linked traits

• Many traits that are carried on the X chromosome do not have a corresponding spot on the Y chromosome. This causes for some unique possibilities for the offspring.

• Ex. Colorblindness in humans, fruit fly eye color, hemophilia.

Pleiotropy

• Condition where a single gene influences multiple traits– Example: Marfan’s Syndrome

Environment

• Phenotype = Genotype + Environment

• Epigenetics – study of changes in phenotype due to mechanisms other than changes in DNA sequence

Epigenetics

Polygenics

• Traits that are determined by many genes – Examples – eye color,

skin color, height, etc.

Analyzing Patterns

• Pedigree – illustration of relationships among family members over multiple generations

Inheritance and Genetic Disorders

• Hereditary disorders– Autosomal or sex-linked– Recessive or dominant

• Genetic disorders are often polygenic and influenced by environment

Autosomal Recessive Disorders• Carriers – heterozygotes that have one allele but not

the disorder– Examples – Cystic Fibrosis, Tay Sachs

Autosomal Dominant Disorders• Disorders that are expressed

with only one copy of the allele– Example – polydactyly – Example – Huntington’s

Sex-linked Recessive Disorders

• Disorders that are on the sex chromosome – Example – Hemophilia

Sex-linked Dominant Disorders