The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment

The alignment of one pair of homologs is independent of any other.

Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

The Punnett Square for a Dihybrid Cross

Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.

Dih

ybri

d C

ross

(2 lo

ci, 2

alle

les) 9:3:3:1 ratio that is dependent on:

• Two loci, two alleles per locus• Independent assortment between loci (genotypic

independence)• Dominance-recessive relationships between the

alleles found at each locus• One locus does not affect the phenotype of the

other locus (phenotypic independence)

3:1 ratios are all over this

Consider a cross between parents heterozygous for both deafness and albinism.

This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

What Works for Peas Also Works for Humans

Some Alleles Are Related Through Incomplete Dominance

Dominance relationships may differ, but the Principle of Segregation remains the same.

Pleiotropy – When One Allele Influences Many Traits

Pleiotropy in Action

Anemia, infections, weakness, impaired growth, liver and spleen failure, death.

Traits (phenotypes) associated with the sickle cell allele.

Polygenic Inheritance – When a Single Trait is Influenced by Many Genes

Height is a polygenic trait

Multiple Alleles

Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles.

The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene.

Codominance

The human ABO blood group illustrates another genetic phenomenon – codominance.

Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote.

The AB phenotype (genotype IA

IB) is an example of codominance

Pedi

gree

Ana

lysi

s

Hum

an T

raits

Dominant Trait Recessive TraitWidow’s peak Straight hairlineFreckles No frecklesFree earlobe Attached earlobeNormal Cystic fibrosisNormal PhenylketonuriaNormal Tay-Sachs diseaseNormal AlbinismNormal hearing Inherited deafnessHuntington’s Disease NormalDwarfism Normal height

Most genetic diseases are recessive traitsIn other words, there is an absence of a protein function

Table is from http://207.233.44.253/wms/reynolmj/lifesciences/lecturenote/bio3/Chap09.ppt

Dominant vs. Recessive

NoYesAt least one parent of affected child must be affected?

YesNoTrait skips generations?

YesYesMales and females transmit the trait?

YesYesMales and females affected?

Autosomal recessive

Autosomal dominant

Note that lethal dominant traits tend to be very rare because affected individuals tend to die before mating

Autosomal Dominant Inheritance

Generations are not skipped

Typically about half the offspring are affected, but don’t count on this!!!

No silent carriers

Autosomal Dominant Inheritance

Generations are not skipped

Autosomal Dominant Inheritance

Generations are not skipped

Huntington’s disease

Pedigree Analysis (dominant)

Autosomal Recessive InheritanceHeterozygotes carry the

recessive allele but exhibit the wildtype phenotype

Males and females are equally affected and may transmit the trait

May skip generationsNote that with rare

recessive traits we usually assume that people from outside of a family do not possess the affecting allele

Autosomal Recessive Inheritance

Generation skipped

Autosomal Recessive Inheritance

Generations skipped

Often both parents are

silent carriers

Typical is 1/4th affected

Sickle-cell disease

Cystic Fibrosis

Con

sang

uine

ous M

atin

g

“With blood”

Inbreeding unmasks

otherwise rare recessive traits

because genotypes of

parents are not independent Consanguineous

mating (=)

Autosomal Recessive InheritanceGenerations

skipped

More likely early onset lethal than if

dominant

Pedigree Analysis (recessive)Generation

skipped

Genetics Problem-Solving Secrets! Known Genotype can be used to infer unknown Phenotype

(but not always, due to complications, e.g., penetrance) Known Phenotype can be used to infer unknown Genotype

(but not always due to lack of 1:1 correspondence: more than one genotype can give rise to a given phenotype)

Genotype (diploid) gives rise to Gametes (haploid) via Meiosis Gametes (haploid) give rise to “Progeny” (diploid) via

Fertilization Fertilization (syngamy) always results in Diploidy (I.e.,

>ploidy than haploid) Meiosis always results in Haploidy (I.e., anaphase I reduction

division from diploidy to haploidy)