Fig. 1 1.1 Gregor Mendel

Mader: Biology 8th Ed.

Mendelian Patterns of Inheritance

Chapter 11

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Outline

• 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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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.

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Fig. 11.1

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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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Garden Peas

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Fig. 11.2

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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 = P

!First generation offspring = F1

!Second generation offspring = F2

– Formulated law of segregation

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Mendel’s Monohybrid Cross

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Mendel’s Monohybrid Cross

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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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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

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Homologous Chromosomes

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Genotype Versus Phenotype

• Genotype

– Refers to the alleles an individual receives at fertilization.

• Phenotype

– Refers to the physical appearance of the individual.

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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.

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Genetic Inheritance in Humans

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Testcross

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

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Testcross

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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

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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.

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Dihybrid Cross

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Dihybrid Cross

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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.

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Two-Trait Testcross

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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.

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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

– 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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Incomplete Dominance

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

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Incomplete Dominance

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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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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)

• 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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Environment and Phenotype

• Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.

– Black fur only occurs on extremities.

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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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Documents

Fig. 1 1.1 Gregor Mendel