Chromosomal Basis of Inheritance

Chapter 15

Objectives• Be familiar with patterns of inheritance for

autosomal and sex linked genes• Understand the concept of “Linked Genes”• Understand how traits affected by incomplete

dominance and codominance differ from autosomal dominant and autosomal recessive traits

• Understand how nondisjunction of chromosomes can lead to disorders.

Linked Genes

• Linked genes are those that reside on the same chromosome and tend to be inherited together

• Humans have 23 pairs of chromosomes– Autosomal genes reside on the autosomal

chromosomes (pairs 1-22)– Sex-linked genes are found on the sex

chromosomes (pair 23, usually on the X)

Karyotype: picture of a complete set of chromosomes

Autosomal Genes

• Autosomal genes are usually represented by a pair of alleles

• The phenotype of the gene reflects the dominant/recessive relationship of the alleles

Autosomal Disorders

• Most autosomal genetic diseases are autosomal recessive meaning the individual need to be homozygous recessive to exhibit the condition (example: cystic fibrosis)

• Huntington’s disease is an autosomal dominant disorder meaning that is a single Huntingtons allele is inherited, the individual will have the disease.

Incomplete and Codominance

• Some alleles do not show a dominance hierarchy

• Incomplete dominance: the phenotype of a heterozygous genotype is intermediate in appearance

• Codominance: each allele in the genotype for a particular gene will be expressed in the phenotype

Incomplete Dominance (heterozygous genotype is intermediate in appearance)

Codominance (each genotypic allele will be expressed)

Sex Linked Inheritance• Males and females differ in

their sex chromosome combination (females XX; males XY)

• Because the X contains genes and the Y “does not”, inheritance patterns of sex-linked genes vary between the sexes – recessive traits more

prevalent in males

Is the Disorder an Autosomal Recessive Disorder or Is It Sex Linked?

Is This Disorder Autosomal Recessive or Sex Linked?

Errors In Chromosomal Inheritance

• Genetic disorders can also occur due to errors in the number of inherited chromosomes

• This condition arises through a problem that occurs during meiosis

Nondisjunction

• Failure of a pair of chromosomes to separate during gamete formation

• Genetic disorders like: – Down’s syndrome (#21, 3n)– Turner syndrome (XO)– Metafemale (XXX)– Kleinfelter Male (XXY)– Jacob’s Male (XYY)

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X inactivation in female mammals

• Although female mammals, including humans, inherit two X chromosomes, one X chromosome in each cell becomes almost completely inactivated during embryonic development.– Barr body

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Example of Barr Body

http://griffing.tamu.edu/BIOETCSITE/Biology01/Module11/Unit8/Barr.htm

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Abnormal Chromosome #

• Nondisjunction– Leads to aneuploidy:

• Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality.

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Abnormal Chromosome #

• Nondisjunction– Leads to aneuploidy:

• Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality.

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Alterations of chromosome structure

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The Key Points in Review

• Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles.

• Morgan traced a gene to a specific chromosome.

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Key Points in Review

• Linked genes tend to be inherited together because they are located on the same chromosome.

• Independent assortment of chromosomes and crossing over produce genetic variation (recombinants)

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Key Points in Review

• Geneticists can use recombination data to map a chromosomes genetic loci.

• Chromosomal basis for sex is dependent upon the organism.

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Key Points for Review

• Sex-linked genes have unique patterns of inheritance.

• Alterations of chromosome numbers or structure cause some genetic disorders.