Ch 13: Chromosomal Inheritance

chromosomal theory of inheritance

• during meiosis, chromosome pairs migrate as discrete structures

  • chromosomes sorting from each homologous pair into pre-gametes appears to be random

• gametes contain only ½ the chromosomal complement of parent

• sperm and egg differ in size and morphology but have the same number of chromsomes

  • suggests equal genetic contributions per parent

• gametic chromosomes combine during fertilization to produce diploid offspring

T.H. Morgan and his work with fruit flies (1900s)

• genetic linkage (in this case linked to the sex chromosomes) was the first demonstration that genes were located on chromosomes

  • white-eyed mutant allele was inherited along with the X chromosome → “X-linked” or “sex-linked”

• considered how linkage affects inheritance of 2 different characteristics

  • linked genes tend to be inherited together and are physically near each other on the same chromosome

• violates the Law of Independent Assortment

What did Morgan determine regarding genetic linkage?

• genes that are close together on same chromosome are linked and do not assort independently

• unllinked genes are either on separate chromosomes or are far apart on same chromosome and assort independently

recombination of unliked genes: independent assortment of chromosomes

when Mendel followed inheritance of 2 characters

• observed some offspring have combinations of traits that do NOT match either parent in P generation (YYRR x yyrr)

recombination of linked genes: crossing over

• recombinant offspring

  • show new combos of parental traits

  • when 50% of offspring are recombinants there is a 50% recombination frequency (RF)

  • linked genes have a RF of less than 50%

    • related to physical distance on the chromosome (more likely to be inherited together)

• Morgan discovered that genes could be linked

  • but appearance of recombinant phenotypes made linkage appear incomplete

  • proposed some processes must occasionally break the physical connection between genes on same chromosomes

    • crossing over of homologous chromosomes

    • lower RF = less distance between traits

linkage mapping: using recombination data

• genetic map

  • an ordered list of the genetic loci along a particular chromosome

  • can be developed using recombination frequencies

• the farther apart genes are on a chromosome, the more likely it is for a crossover event to occur between them

• genes with the least amount of distance between them are most linked

recombination frequency cannot be more than what percentage?

50

aneuploidy

• results from the fertilization of gametes in which nondisjunction occurred

• is a condition in which offspring have an abnormal number of a particular chromosome

  • usually for one chromosome (if more, organism most likely wont survive

euploidy

having the normal number of chromosomes

monosomy

missing a partner for a chromosome

trisomic

3 copies of a particular chromosome

monosomic

1 copy of a particular chromosome

when does nondisjunction occur?

when chromosomes or sister chromatids fail to separate during meiosis I or meiosis II, resulting in an abnormal chromosome number

trisomy 21

nondisjunction on the 21st chromosome, the incidence of having offspring with trisomy 21 increase dramatically with maternal age

polyploidy for specific plants and animals

• this is their normal number

• condition in which there are more than 2 complete sets of chromosomes in an organism

sex chromosome nondisjunction example

• in cats, the gene for coat color is located on the X chromosome (sex-linked, mainly in females)

• in the embryonic development of female cats, one of the two X chromosomes is randomly inactivate in each cell, resulting in a tortoiseshell pattern if the cat has two different alleles for coat color

• male cats, having only one X chromosome, never exhibit a tortoise shell color

alterations of chromosome structure

breakage of a chromosome can lead to 4 types of changes in chromosome structure

• deletion

• duplication

• inversion

• translocation

deletion

removes a chromosomal segment

duplication

repeats a segment

inversion

reverses a segment within a chromosome

translocation

moves a segment from one chromosome to another nonhomologous one

reciprocal translocation

most common type: nonhomologous chromosomes exchange fragments

nonreciprocal translocation

chromosome transfers a fragment without receiving a fragment in return

Klinefelter syndrome

• result of extra X chromosome in a male, producing XXY individuals (likely came from mom)

• have male sex organs, but sterile, some breast development

Turner syndrome

• result of monosomy X, producing an X O karyotype

• sterile, but phenotypically normal if given hormone therapy

Cri du chat

• not nondisjunction

• caused by deletion in chromosome 5

• causes sever mental retardation

chronic myelogenous leukemia (CML)

cancer results

the inheritance of traits controlled by genes present in the chloroplasts or mitochondria

depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg

some disease affecting the muscular and nervous systems

• caused by defects in mitochondrial genes that prevent cells from making enough ATP → no energy → chronic fatigue

• mitochondrial myopathy

• Leber’s hereditary optic neuropathy