Ch. 15 The Chromosomal Basis of Inheritance

Ch. 15 The Chromosomal Basis of Inheritance

15.3-15.515.3-15.5

Main IdeasMain Ideas

1. Sex-linked inheritance and issues

2. Aneuploidy3. Chromosomal mutations4. Genomic imprinting5. Non-nuclear DNA

1. Sex-linked inheritance and issues

2. Aneuploidy3. Chromosomal mutations4. Genomic imprinting5. Non-nuclear DNA

Sex-linked genes exhibit unique patterns of inheritance

Sex-linked genes exhibit unique patterns of inheritance

The XX, XY system in humans

The XX, X system in grasshoppers

The ZW, ZZ system in chickens

The diploid, haploid system in bees

The XX, XY system in humans

The XX, X system in grasshoppers

The ZW, ZZ system in chickens

The diploid, haploid system in bees

Sex Chromosomes in HumansSex Chromosomes in Humans Females - XX Males XY

The Y chromosome is homologous with certain regions on the X chromosome

Parents pass on one sex chromosome to their children through their gametes Woman always pass on an X

chromosome Males pass on either an X (to make

a daughter) or a Y (to make a son)

Females - XX Males XY

The Y chromosome is homologous with certain regions on the X chromosome

Parents pass on one sex chromosome to their children through their gametes Woman always pass on an X

chromosome Males pass on either an X (to make

a daughter) or a Y (to make a son)

Sex Chromosomes in HumansSex Chromosomes in Humans Early in development,

the embryo has “generic” sex parts

Whether these parts develop into testes or ovaries is determined by the presence of hormones within the embryo

The “default” is a female

Early in development, the embryo has “generic” sex parts

Whether these parts develop into testes or ovaries is determined by the presence of hormones within the embryo

The “default” is a female

Weeks of development

On the Y chromosome is a gene called the SRY gene (sex-determining region of Y)

This gene codes for a protein that triggers other genes, leading to the development of a male

On the Y chromosome is a gene called the SRY gene (sex-determining region of Y)

This gene codes for a protein that triggers other genes, leading to the development of a male

Quick ThinkQuick Think

Why do you think it is the presence of a Y chromosome that makes someone male,

and not the presence of two X chromosomes that makes

someone female? (i.e. - why is female the default)?

Why do you think it is the presence of a Y chromosome that makes someone male,

and not the presence of two X chromosomes that makes

someone female? (i.e. - why is female the default)?

Inheritance of Sex-linked GenesInheritance of Sex-linked Genes Sex-linked genes - genes on the sex

chromosomes (may or may not be related to gender)

We see different inheritance patterns in males and females because females have 2 X chromosomes where males only have 1

Sex-linked genes - genes on the sex chromosomes (may or may not be related to gender)

We see different inheritance patterns in males and females because females have 2 X chromosomes where males only have 1

X-linked traitsX-linked traits Women have 2 X-

chromosomes, men have an X and a Y.

For women to express a For women to express a recessive phenotype, they recessive phenotype, they must inherit 2 X-must inherit 2 X-chromosomes, both with the chromosomes, both with the recessive allele.recessive allele.

For men to express the For men to express the recessive phenotype, they recessive phenotype, they need only 1 recessive Xneed only 1 recessive X

Women have 2 X-chromosomes, men have an X and a Y.

For women to express a For women to express a recessive phenotype, they recessive phenotype, they must inherit 2 X-must inherit 2 X-chromosomes, both with the chromosomes, both with the recessive allele.recessive allele.

For men to express the For men to express the recessive phenotype, they recessive phenotype, they need only 1 recessive Xneed only 1 recessive X

ColorblindnessColorblindness Colorblindness is recessive and is a mutation

of a gene on the X chromosome Men are more likely to be colorblind than

women…WHY

Colorblindness is recessive and is a mutation of a gene on the X chromosome Men are more likely to be colorblind than

women…WHY

Duchenne muscular dystrophyDuchenne muscular dystrophy

1/3500 males in USProgressive weakening

of musclesLoss of coordinationDeath by early 20sMutation in X

chromosome in the gene that codes for a muscle protein (dystrophin)

1/3500 males in USProgressive weakening

of musclesLoss of coordinationDeath by early 20sMutation in X

chromosome in the gene that codes for a muscle protein (dystrophin)

HemophiliaHemophilia

Prolonged bleeding when injured Clots slow to formCaused by a mutation for a gene on the

X chromosome that codes for a blood clotting protein

Can be treated but not cured

Prolonged bleeding when injured Clots slow to formCaused by a mutation for a gene on the

X chromosome that codes for a blood clotting protein

Can be treated but not cured

Bleeding (bruising) after an injection

X inactivation in females

X inactivation in females

Even though females get 2 X chromosomes, 1 becomes inactive early in the development of the embryo

The inactive X condenses into a Barr body

This means that the cells of males and females each have 1 active X chromosome

Even though females get 2 X chromosomes, 1 becomes inactive early in the development of the embryo

The inactive X condenses into a Barr body

This means that the cells of males and females each have 1 active X chromosome

Which X becomes inactive in each embryonic cell is random In some cells, the paternal X will do this In other cells, the maternal X will do this

This leads to mosaicism in females All cells that developed from the embryonic cells

containing the maternal X will exhibit certain traits The cells that develop from the embryonic cells

containing the paternal X will exhibit other traitsExample: tortoiseshell coat color in cats

Which X becomes inactive in each embryonic cell is random In some cells, the paternal X will do this In other cells, the maternal X will do this

This leads to mosaicism in females All cells that developed from the embryonic cells

containing the maternal X will exhibit certain traits The cells that develop from the embryonic cells

containing the paternal X will exhibit other traitsExample: tortoiseshell coat color in cats

Quick ThinkQuick Think

Can a male cat be a calico or tortoiseshell?

Explain your reasoning.

Can a male cat be a calico or tortoiseshell?

Explain your reasoning.

Abnormal Chromosome NumbersAbnormal Chromosome Numbers

Occasionally, the chromosomes do not segregate properly during meiosis This is called nondisjunction This results in some gametes with too

many chromosomes and some gametes with too few

Occasionally, the chromosomes do not segregate properly during meiosis This is called nondisjunction This results in some gametes with too

many chromosomes and some gametes with too few

Having an abnormal number of chromosomes is a condition known as aneuploidy

Having 3 copies of a particular chromosome is called a trisomy

Having 1 copy of a particular chromosome is called a monosomy

Having an abnormal number of chromosomes is a condition known as aneuploidy

Having 3 copies of a particular chromosome is called a trisomy

Having 1 copy of a particular chromosome is called a monosomy

Down SyndromeDown Syndrome

Caused by a trisomy of chromosome 211/700 births in USThe result of nondisjunction during meiosis IRisk increases with the age of the motherMental retardation, increased risk of other

diseases, shortened life span, underdeveloped and likely sterile

Caused by a trisomy of chromosome 211/700 births in USThe result of nondisjunction during meiosis IRisk increases with the age of the motherMental retardation, increased risk of other

diseases, shortened life span, underdeveloped and likely sterile

Trisomy 18 – Edward’s syndrome

low birth weight, mental retardation,

extra fingers and toes

Trisomy 18 – Edward’s syndrome

low birth weight, mental retardation,

extra fingers and toes

Abnormal Sex ChromosomesAbnormal Sex Chromosomes

Extra or missing sex chromosomes also cause abnormal phenotypes.

If an individual only has one sex chromosome, they have Turner syndrome (45, X karyotype).

These people are short, underdeveloped, have a web neck, and sterile females.

Extra or missing sex chromosomes also cause abnormal phenotypes.

If an individual only has one sex chromosome, they have Turner syndrome (45, X karyotype).

These people are short, underdeveloped, have a web neck, and sterile females.

Abnormal Sex chromosomesAbnormal Sex chromosomesXXY syndrome (47,

XXY karyotype) is called Klinefelter syndrome. These males are very tall and sexually underdeveloped, with diminished intelligence and some female phenotypes due to the extra X chromosome

XXY syndrome (47, XXY karyotype) is called Klinefelter syndrome. These males are very tall and sexually underdeveloped, with diminished intelligence and some female phenotypes due to the extra X chromosome

Polyploidy - having more than two complete chromosome sets The normal state is having 2 of each chromosome

(homologous pairs) When organisms have 3 or 4 of each chromosome, that is

called polyploidyTriploidy (3n), tetraloidy (4n), hexploid (6n), octapolid (8n)

Very uncommon in animals Some fish and amphibians, mostly

Quite common in plants

Polyploidy - having more than two complete chromosome sets The normal state is having 2 of each chromosome

(homologous pairs) When organisms have 3 or 4 of each chromosome, that is

called polyploidyTriploidy (3n), tetraloidy (4n), hexploid (6n), octapolid (8n)

Very uncommon in animals Some fish and amphibians, mostly

Quite common in plants

Recently discovered tetraploid mammal, rodent from Chile

Quick ThinkQuick Think

Gene dosage, the number of active copies of a gene, is important to proper development.

Identify and describe two disorders that are the result of improper gene dosage

Gene dosage, the number of active copies of a gene, is important to proper development.

Identify and describe two disorders that are the result of improper gene dosage

Alterations of Chromosome StructureAlterations of Chromosome StructureChromosomal mutations

include Deletion Inversion Duplication Translocation

Often occur during meiosis Chromosomes break &

rejoin incorrectly

Chromosomal mutations include Deletion Inversion Duplication Translocation

Often occur during meiosis Chromosomes break &

rejoin incorrectly

Deletion mutationsDeletion mutations

A piece of chromosome is lost during meiosis

Cri du chat is one condition that results - distinctive cry, severe retardation, shortened life span

A piece of chromosome is lost during meiosis

Cri du chat is one condition that results - distinctive cry, severe retardation, shortened life span

Inversion MutationsInversion Mutations

A double break where the piece reattaches, but backwards

Hemophilia is a inversion on the X chromosome

A double break where the piece reattaches, but backwards

Hemophilia is a inversion on the X chromosome

Duplication mutation Duplication mutation

A piece breaks off from one chromosome and reattaches to the sister chromatid

Fragile X syndrome is caused by this, 2nd most common form of mental retardation after Downs syndrome

A piece breaks off from one chromosome and reattaches to the sister chromatid

Fragile X syndrome is caused by this, 2nd most common form of mental retardation after Downs syndrome

Translocation mutationTranslocation mutation

A piece breaks off one chromosome and reattaches to a different chromosome

Burkitt’s lymphoma is caused by this

A piece breaks off one chromosome and reattaches to a different chromosome

Burkitt’s lymphoma is caused by this

Quick Think Quick Think

At what point in gamete formation would these chromosomal mutations occur? During what particular process might the chromosomes be especially vulnerable to these mutations?

At what point in gamete formation would these chromosomal mutations occur? During what particular process might the chromosomes be especially vulnerable to these mutations?

Exceptions to the chromosome theory of inheritance

Exceptions to the chromosome theory of inheritance Sometimes, the phenotype

of the individual is different depending on which parent passed along the particular allele Example: in mice, there is a

normal lgf2 gene and a mutant lgf2 gene

If the mother gives mutant & the father gives normal, the mouse grows to normal size

But, if the father gives a mutant allele, the mouse will be dwarf

This is called genomic imprinting

Sometimes, the phenotype of the individual is different depending on which parent passed along the particular allele Example: in mice, there is a

normal lgf2 gene and a mutant lgf2 gene

If the mother gives mutant & the father gives normal, the mouse grows to normal size

But, if the father gives a mutant allele, the mouse will be dwarf

This is called genomic imprinting

In genomic imprinting – one copy of a gene is silenced during gamete formation by the addition (usually) of a methyl (--CH3) group to the cytosine nucleotides of that gene

Occurs in a small number of genes critical to embryonic development in animals

Therefore ONLY the maternal or paternal copy of that gene can be expressed in the offspring

In genomic imprinting – one copy of a gene is silenced during gamete formation by the addition (usually) of a methyl (--CH3) group to the cytosine nucleotides of that gene

Occurs in a small number of genes critical to embryonic development in animals

Therefore ONLY the maternal or paternal copy of that gene can be expressed in the offspring

DNA in organellesDNA in organelles

DNA is also found in DNA is also found in mitochondriamitochondria and chloroplasts. and chloroplasts.

This DNA is not passed to This DNA is not passed to gametes through meiosis like gametes through meiosis like nuclear DNA is.nuclear DNA is.

Mitochondrial DNA is only Mitochondrial DNA is only passed from Mother to child.passed from Mother to child.

The genes in mitochondrial DNA The genes in mitochondrial DNA code for proteins of the ETC & code for proteins of the ETC & ATP synthaseATP synthase Mutations in this DNA may Mutations in this DNA may

contribute to nervous system contribute to nervous system disorders, diabetes, heart disease, disorders, diabetes, heart disease, & Alzheimer’s & Alzheimer’s

DNA is also found in DNA is also found in mitochondriamitochondria and chloroplasts. and chloroplasts.

This DNA is not passed to This DNA is not passed to gametes through meiosis like gametes through meiosis like nuclear DNA is.nuclear DNA is.

Mitochondrial DNA is only Mitochondrial DNA is only passed from Mother to child.passed from Mother to child.

The genes in mitochondrial DNA The genes in mitochondrial DNA code for proteins of the ETC & code for proteins of the ETC & ATP synthaseATP synthase Mutations in this DNA may Mutations in this DNA may

contribute to nervous system contribute to nervous system disorders, diabetes, heart disease, disorders, diabetes, heart disease, & Alzheimer’s & Alzheimer’s