Linked Genes, X-linked inheritance, and Mutations

Linked Genes, X-linked inheritance, and Mutations

Ch. 15Ch. 15

Genes are on ChromosomesGenes are on

Chromosomes

• This is obvious to us, but it has only been known for a few decades

• This is obvious to us, but it has only been known for a few decades

Chromosome Theory of

Inheritance

Chromosome Theory of

Inheritance• The work of scientists in the early

1900s• Says 2 things:

1. Genes occupy specific loci (positions on chromosomes)

2. Chromosomes undergo segregation and independent assortment during meiosis

• The work of scientists in the early 1900s

• Says 2 things:1. Genes occupy specific loci (positions on

chromosomes)2. Chromosomes undergo segregation and

independent assortment during meiosis

Thomas Hunt Morgan and his fruit flies

Thomas Hunt Morgan and his fruit flies

• The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

• The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

Fruit flies were a good choice…

Fruit flies were a good choice…

• They breed at a high rate

• A generation can be bred every two weeks

• They have only four pairs of chromosomes

• They breed at a high rate

• A generation can be bred every two weeks

• They have only four pairs of chromosomes

What Morgan did…What Morgan did…

• He spent a year looking for variant individuals, those that differed from the normal, or “wild” phenotype

• Traits alternative to the wild type are called mutant phenotypes

• He spent a year looking for variant individuals, those that differed from the normal, or “wild” phenotype

• Traits alternative to the wild type are called mutant phenotypes

He found a white eyed

male…

He found a white eyed

male…• He mated it to a “wild” type red eyed female and

got all red eyed offspring in the F1 generation• What does that tell us?

• He let the F1s mate, and got the classic 3:1 ratio of red:white eyes, but only in males• What does that tell us?

• Morgan figured out that the gene for eye color was on the X chromosome

• His work supported the chromosome theory of inheritance

• He mated it to a “wild” type red eyed female and got all red eyed offspring in the F1 generation• What does that tell us?

• He let the F1s mate, and got the classic 3:1 ratio of red:white eyes, but only in males• What does that tell us?

• Morgan figured out that the gene for eye color was on the X chromosome

• His work supported the chromosome theory of inheritance

Morgan’s other workMorgan’s other work

• Each chromosome has hundreds or thousands of genes

• Genes located on the same chromosome that tend to be inherited together are called linked genes

• Each chromosome has hundreds or thousands of genes

• Genes located on the same chromosome that tend to be inherited together are called linked genes

Morgan experimented to see inheritance of two traits

Morgan experimented to see inheritance of two traits

• Morgan crossed flies that differed in traits of body color and wing size

• Morgan crossed flies that differed in traits of body color and wing size

What he found…What he found…

• He found that the body color and wing size traits were usually inherited together and the offspring looked like the parents, mostly…

• He found that the body color and wing size traits were usually inherited together and the offspring looked like the parents, mostly…

What he found…What he found…• Some of the offspring had phenotypes different

from the parents, but in lower ratios than expected• Offsrping with phenotypes like the parents called

parental types• Offspring with new phenotype combos called

recombinant types• 50% frequency of recombination = genes on 2

different chromosomes

• Some of the offspring had phenotypes different from the parents, but in lower ratios than expected

• Offsrping with phenotypes like the parents called parental types

• Offspring with new phenotype combos called recombinant types

• 50% frequency of recombination = genes on 2 different chromosomes

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-roundheterozygous parent (YyRr)

Parental-typeoffspring

Recombinantoffspring

What he found…

What he found…

• He discovered that some genes can be linked, but there is a process that can break their connection: crossing over

• This led to the development of genetic maps that determine the location of each gene on a chromosome based on the frequency of recombination

• Distance between genes expressed as map units• 1 map unit = 1% recombination frequency

• He discovered that some genes can be linked, but there is a process that can break their connection: crossing over

• This led to the development of genetic maps that determine the location of each gene on a chromosome based on the frequency of recombination

• Distance between genes expressed as map units• 1 map unit = 1% recombination frequency

Sex-linked genes

Sex-linked genes

• An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes

• Mammals like humans have an XX or XY system of inheritance

• Other organisms have other systems

• An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes

• Mammals like humans have an XX or XY system of inheritance

• Other organisms have other systems

Genes on the sex chromosomes are called sex-linked

genes

Genes on the sex chromosomes are called sex-linked

genes• Some diseases on the X chromosome:• Color blindness

• Rare in females, mild disease• Duchenne muscular dystrophy

• 1 in 3500 males in US gets it• Lack the gene for the muscle

protein dystrophin• Muscles get weaker and lose

coordination• Usually don’t live past 20s

• Hemophilia• Lack the protein to cause clotting• Don’t clot normally

• Some diseases on the X chromosome:• Color blindness

• Rare in females, mild disease• Duchenne muscular dystrophy

• 1 in 3500 males in US gets it• Lack the gene for the muscle

protein dystrophin• Muscles get weaker and lose

coordination• Usually don’t live past 20s

• Hemophilia• Lack the protein to cause clotting• Don’t clot normally

Barr bodiesBarr bodies

• In mammalian females, 1 of the 2 X chromosomes is inactivated during embryonic development

• The inactive X condenses into what is called a Barr body (we can see it under the microscope)

• If she is heterozygous for a sex-linked trait, she will be a mosaic for that trait

• In mammalian females, 1 of the 2 X chromosomes is inactivated during embryonic development

• The inactive X condenses into what is called a Barr body (we can see it under the microscope)

• If she is heterozygous for a sex-linked trait, she will be a mosaic for that trait

• Some cells have the maternal X inactivated• These cells have

the orange color

• Some cells have the paternal X inactivated• These cells have

the black color

• All cells in the ovaries have active X chromosomes

• Some cells have the maternal X inactivated• These cells have

the orange color

• Some cells have the paternal X inactivated• These cells have

the black color

• All cells in the ovaries have active X chromosomes

Chromosomal mutations

Chromosomal mutations

• In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

• In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

What results…What results…

• Aneuploidy - a zygote produced from a normal gamete and a gamete produced by nondisjunction • Offspring with this

condition have an abnormal number of a particular chromosome

• Aneuploidy - a zygote produced from a normal gamete and a gamete produced by nondisjunction • Offspring with this

condition have an abnormal number of a particular chromosome

What results…What results…

• Trisomy - having 3 copies of a particular chromosome

• Monosomy - having just one copy of a particular chromosome

• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

• Trisomy - having 3 copies of a particular chromosome

• Monosomy - having just one copy of a particular chromosome

• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

Recent research has shown that this Chilean rodent is a tetraploid

Very rare among animals

Common in plants, some fish, some amphibians

Chromosomal breakage

Chromosomal breakage

• Breakage of a chromosome can lead to four types of changes in chromosome structure:• Deletion removes a chromosomal

segment

• Breakage of a chromosome can lead to four types of changes in chromosome structure:• Deletion removes a chromosomal

segment

Deletion mutationDeletion mutation

• Example: retinoblastoma (eye tumors)

• Example: retinoblastoma (eye tumors)

Chromosomal breakage

Chromosomal breakage

• Duplication repeats a segment• Duplication repeats a segment

Duplication mutation – fragile X syndrome Duplication mutation – fragile X syndrome

Chromosomal breakage

Chromosomal breakage

• Inversion reverses a segment within a chromosome

• Inversion reverses a segment within a chromosome

Hemophilia A – inversion mutation patient was given

injection in buttocks

Hemophilia A – inversion mutation patient was given

injection in buttocks

Chromosomal breakage

Chromosomal breakage

• Translocation moves a segment from one chromosome to another

• Translocation moves a segment from one chromosome to another

Translocation mutation causes Burkitt’s

lymphoma Tumors on hand from cancer

Translocation mutation causes Burkitt’s

lymphoma Tumors on hand from cancer

Down syndromeDown syndrome

• Trisomy 21 - 3 number 21 chromosomes

• 1 in 700 children in US

• Frequency increases with age of mother

• Trisomy 21 - 3 number 21 chromosomes

• 1 in 700 children in US

• Frequency increases with age of mother

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

Trisomy of sex chromosomes

Trisomy of sex chromosomes• Klinefelter

syndrome is the result of an extra chromosome in a male, producing XXY individuals

• Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals

• Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans

• Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans