Chapter11

Mader: Biology 8th Ed.

Mendelian Patterns of Inheritance

Chapter 11

Mader: Biology 8th Ed.

Blending Inheritance

• At the time, most breeders believed parents of contrasting appearance always produce offspring of intermediate appearance.

• Mendel’s experiments helped him formulate the particulate theory of inheritance.– Inheritance involves reshuffling of genes

from generation to generation.

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Law of Segregation

• Each individual has two factors for each trait.• The factors segregate during gamete

formation.• Each gamete contains only one factor from

each pair of factors.• Fertilization gives each new individual two

factors for each trait.

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Law of Segregation Explained

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Homologous Chromosomes

Law of segregation applies to alleles on SEPARATE chromosomes.

Alleles that tend to be inherited together are said to be linked.

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Law of Independent Assortment

• Each pair of factors segregates independently of the other pairs.

• All possible combinations of factors can occur in the gametes.

• Possible combinations = 2n – where n = number of heterozygous

chromosome pairs

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Punnett Square

• Tool for determining all possible offspring combinations

– Every possible combination of alleles is placed within the squares.

– Single-Trait Cross means there will be two letters in each offspring’s square-one from mom and one from dad

X

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Dihybrid Cross: Two-trait Inheritance

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Determining possible gametes…

Identify parents’ gametes:

Bbmm BbMm

Bm BMBm Bmbm bMbm bm

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Building the Punnett Square

Bm Bm bm bm

BM BBMm BBMm BbMm BbMm

Bm BBmm BBmm Bbmm Bbmm

bM BbMm BbMm bbMm bbMm

bm Bbmm Bbmm bbmm bbmm

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Incomplete Dominance

• Exhibited when the heterozygote has an intermediate phenotype between that of either homozygote.

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Multiple Allelic Traits

• When a trait is controlled by multiple alleles, the gene exists in several allelic forms.

– ABO blood types IA – A antigens on blood cells IB – B antigens on blood cells i - no antigens on blood cells

• Codominance - More than one allele is fully expressed.

– ABO blood type (multiple allelic traits)

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Multiple Allelic Traits

• When a trait is controlled by multiple alleles, the gene exists in several allelic forms.– ABO blood types

Phenotype Genotype

A IAIA,IAi

B IBIB,IBi

AB IAIB

O ii

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Inheritance of Blood Type

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Hardy-Weinberg Principle

** Allele frequencies of gene pool will stay the same (equilibrium) IF

–No Mutations–No Gene Flow–Random Mating–No Genetic Drift–No Selection

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•Consider a trait with 2 possible alleles…

p = frequency of dominant alleleThe probability that an allele chosen at random is dominantq = frequency of recessive alleleThe probability that an allele chosen at random is recessivep + q = 1; Why?

Hardy Weinberg Math

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Hardy-Weinberg Math

What is the probability of an individual being homozygous dominant (AA) – the frequency

of the AA genotype?

fAA = p x p = p2

What is the probability of an individual being homozygous recessive (aa)?

faa = q x q = q2

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Hardy-Weinberg Math

• What is the probability of an individual being heterozygous (Aa) – the frequency of the Aa genotype?

• fAa = (p x q) + (q x p) = 2pq

• There are 2 possible combinations: – allele 1 can be A and allele 2 can be a

OR – allele 1 can be a and allele 2 can be A

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Human Genetic Disorders

• Autosome - Any chromosome other than a sex chromosome.

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Human Genetic Disorders

• Autosome - Any chromosome other than a sex chromosome.– When a genetic disorder is autosomal

dominant, an individual with AA or Aa has the disorder.

– When a genetic disorder is autosomal recessive, only aa individuals have the disorder.

Carriers - Individuals unaffected by a disorder but can have an affected child (carries the recessive allele)

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Autosomal Recessive Pedigree

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Autosomal Dominant Pedigree

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Autosomal Recessive Disorders

• Tay-Sachs Disease– Progressive deterioration of psychomotor

functions.• Cystic Fibrosis

– Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous.

• Phenylketonuria (PKU)– Lack enzyme for normal metabolism of

phenylalanine.

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Autosomal Dominant Disorders

• Neurofibromatosis– Tan or dark spots develop on skin and

darken. Small, benign tumors may arise from fibrous nerve coverings.

• Huntington Disease– Neurological disorder leading to

progressive degeneration of brain cells, in turn causing severe muscle spasms and personality disorders.

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Polygenic Inheritance

• Occurs when a trait is governed by two or more sets of alleles.– Each dominant allele has a quantitative

effect on the phenotype, and these effects are additive.

Result in continuous variation of phenotypes.

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Height in Human Beings

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Terminology

• Pleiotropy - A gene that affects more than one characteristic of an individual.

Sickle-cell (incomplete dominance)• Codominance - More than one allele is fully

expressed.– ABO blood type (multiple allelic traits)

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Terminology

• Epistasis - A gene at one locus interferes with the expression of a gene at a different locus.– Human skin color (polygenic inheritance)

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Epistasis and ABO Blood Group

• In order to produce A and B antigens, a person must have H antigen precursor.

• If both copies of H antigen are mutated, A and B antigens cannot be made.

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Environment and Phenotype

• Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.– Black fur only occurs on extremities.