Mader: Biology 8 th Ed. Mendelian Patterns of Inheritance Chapter 11

Mader: Biology 8th Ed.

Mendelian Patterns of Inheritance

Chapter 11


Outline• Blending Inheritance• Monohybrid Cross

– Law of Segregation• Modern Genetics

– Genotype vs. Phenotype– Punnett Square

• Dihybrid Cross– Law of Independent Assortment

• Human Genetic Disorders


Gregor Mendel

• Austrian monk who formulated fundamental laws of heredity in early 1860s.– Studied science and mathematics at

University of Vienna.– Conducted breeding experiments with the

garden pea Pisum sativum.Carefully gathered and documented

mathematical data from his experiments.


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.


Garden Peas


One-Trait Inheritance

• Mendel performed cross-breeding experiments between true-breeding plants.– Chose varieties that differed in only one

trait (monohybrid cross)– Performed reciprocal crosses

Parental generation = PFirst generation offspring = F1

Second generation offspring = F2

– Formulated law of segregation


Mendel’s Monohybrid Cross


Mendel’s Monohybrid Cross


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.


Modern Genetics View

• Each trait in a pea plant is controlled by two alleles (alternate forms of a gene).

• Dominant allele (capital letter) masks the expression of the recessive allele (lower-case).

• Alleles occur on a homologous pair of chromosomes at a particular gene locus.– Homozygous = identical alleles– Heterozygous = different alleles


Homologous Chromosomes


Genotype Versus Phenotype

• Genotype – Refers to the alleles an individual receives

at fertilization.• Phenotype

– Refers to the physical appearance of the individual.


Punnett Square

• All possible kinds of sperm are lined up on one axis, and all possible kinds of eggs are lined up on the other axis.– Every possible combination of alleles is

placed within the squares.


Genetic Inheritance in Humans


Testcross

• A monohybrid testcross is used to determine if an individual with the dominant phenotype is homozygous dominant or heterozygous for a particular trait.


Testcross


Two-Trait Inheritance

• Mendel performed cross using true-breeding plants differing in two traits.– Dihybrid Cross

Observed phenotypes among F2 plants. Formulated law of independent

assortment


Law of Independent Assortment

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

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


Dihybrid Cross


Dihybrid Cross


Two-Trait Testcross

• A two-trait testcross is used to determine if an individual is homozygous dominant or heterozygous for either of the two traits.


Two-Trait Testcross


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.


Autosomal Recessive Pedigree


Autosomal Dominant Pedigree


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– Lack enzyme for normal metabolism of

phenylalanine.


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.


Incomplete Dominance

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


Incomplete Dominance


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


Inheritance of Blood Type


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.


Height in Human Beings


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)

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


Environment and Phenotype

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


Review• Blending Inheritance• Monohybrid Cross

– Law of Segregation• Modern Genetics

– Genotype vs. Phenotype– Punnett Square

• Dihybrid Cross– Law of Independent Assortment

• Human Genetic Disorders


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Mader: Biology 8 th Ed. Mendelian Patterns of Inheritance Chapter 11