ap bio ch. 15, part 1
TRANSCRIPT
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