ch 15: the chromosomal basis of inheritance

where are genes located?

on chromosomes

when do chromosomes duplicate?

before cell division

how many versions of a gene does each chromosome have?

one (one allele)

law of segregation

two alleles for each gene separate

law of independent assortment

alleles of genes on nonhomologous chromosomes assort independently

why did morgan choose to use fruit flies for his experiment?

1. many offspring

2. short generation time (every two weeks)

3. only 4 pairs of chromosomes

what did morgan find?

all of the white eyed flies were males

how many types of sex chromosomes do humans have?

two: a larger X chromosome and a smaller Y chromosome

(XX→ typically female, XY → typically male)

SRY gene

sex-determining region on the Y gene: triggers testis development in an embryo; located on the Y chromosome

how do X and Y chromosomes pair during meiosis?

they share small homologous regions that allow for pairing

sex-linked gene

gene located on a sex chromosome

x-linked gene

gene located on the x-chromosome

y-linked gene

gene located on the y-chromosome

x-linked recessive traits - female (XX)

usually needs two recessive alleles to express the trait; heterozygous female = carrier

x-linked recessive traits - male (XY)

needs only one recessive allele on X; hemizygous

who are x-linked recessive traits more common in?

males

who do fathers pass their X chromosome to?

daughters (NOT sons)

what happens to an inactive X chromosome?

condenses into a barr body

XX cell → random X inactivation (one active X + one inactive X) → barr body → mosaic expression

which chromosome does NOT have hundreds/thousands of genes?

the Y chromosome

linked genes

genes located on the same chromosome; inherited together (the closer, the more likely of inheritance together)

how does linkage affect inheritance?

1. genes do not assort independently

2. they are linked on the same chromosome

3. crossing over can produce recombinant offpring

how can linked genes be separated?

by crossing over (between homologous chromosomes during prophase I)

recombination of linked genes - crossing over

linked genes usually stay together → crossing over can separate them → recombinant gametes → recombinant offspring

genetic recombination

produces new combos of alleles in offspring

parental type

allele combos found in parents

recombinant types

new allele combos

unlinked genes

50% recombinant offspring

what can chromosomal alterations involve?

chromosome number

chromosome structure

nondisjunction

chromosomes fail to separate normally during meiosis → one gamete receives two of the same type of chromosome while the other receives no copy

when can nondisjunction happen?

1. meiosis I: homologous chromosomes fail to separate → all 4 gametes = abnormal

2. meiosis II: sister chromatids fail to separate → 2 abnormal + 2 normal

aneuploidy

abnormal number of a particular chromosome; nondisjunction → abnormal gametes (n+1 or n-1) → aneuploid zygote

what are some examples of aneuploidy?

trisomy (2n+1) = three copies

monosomy (2n-1) = one copy

polyploidy

more than two complete sets of chromosomes; common in plants, rare in animals; “normal” appearance

what are some examples of polyploidy?

triploidy (3n) = three sets of chromosomes

tetraploidy (4n) = four sets of chromosomes

chromosome structural change: deletion

a chromosomal segment is removed/lost

chromosome structural change: duplication

a segment is repeated

chromosome structural change: inversion

a segment is reversed within a chromosome

chromosome structural change: translocation

a segment from one chromosome is moved to a nonhomologous chromosome

what are some examples of when the parent that contributes the allele matters?

genomic imprinting and organelle inheritance

genetic imprinting

same allele → different effect depending on parent of origin; one parental allele is silenced while the other is expressed

inheritance of organelle genes

mitochondria and chloroplasts contain their own DNA located outside the nucleus

in most animals, who is the mitochondrial DNA primarily inherited from?

the mother