Human Genetics Flashcards
Linked genes
two loci that lie close to one another on the same chromosome
Mendel’s law of Independent assortment is not always accurate–why?
Crossovers and Recombinations
Mendel’s law of Independent assortment
states that gene pairs
assort independently of other gene pairs
Thomas Morgan’s experiment
Mated a gray, normal wing fly (BbVv) which came from a
homozygous dominant parent and a homozygous recessive
parent, with a black, vestigal wing fly (bbvv).
Expected 1:1:1:1
However- saw
185 black normal, 965 gray normal, 944 black vestigial,
206 gray vestigial
• Concluded that these genes are located on the same
chromosome. Therefore,
• Some Bv and bV gametes were formed because of
recombination.
BbVv only gave 2 parental
gametes–BV and bv
recombination–
crossing over to make
combinations that weren’t originally
there
Calculating the Frequency of Crossing over
You can use your data of phenotypes to figure out the percentage of
crossing over between loci.
- Add recombinants
- Divide by TOTAL # of offspring and multiply by 100
- We can only observe the frequency of crossovers, not the actual amount.
when is crossing over more likely to occur? how is this info useful?
when the two loci are farther apart rather than closer together
can therefore generate a genetic map of the chromosome based on the
frequencies or cross-overs between genes.
- Ex- if % of crossing over between A and B is 5%, and between B
and C is 3% A-B 5 map
units
B-C 3 map
units
The actual amount of crossovers is more than we see because
there can be crossing over between the two loci which does not change the combination of genes
X- linked genes
sex-linked genes.
Genes that are only located on the X chromosome, including color perception and blood clotting. X chromosome has 2,062 genes- some associated w sexual and some nonsexual
holandric
Genes on the Y chromosome but not the X chromosome (330)
ex–hairy earlobes, SRY gene–sex
determining region of y chromosome l
female receives (x,y)
x from mother and x from father
male receives (x,y)
x from mother and y from father
hemizygous
(neither homozygous nor heterozygous- only has one copy of the X)- will always express whatever allele is present on x chromosome.
male.
Dosage compensation
A male has only one copy (dose) of the X chromosome and a female has
two (doses).
Male fruit flies makes these two “doses” equivalent by doubling metabolic activity of 1 chromosome
- Mammals accomplish this by inactivating one of the
x chromosomes in every cell of female body
Barr body
inactivated x chromosome; visible as dark spot on edge of nucleus
Dosage compensation–heterozygous females
A female that is heterozygous for a certain X-linked trait has one allele expressed in half of her cells and other allele expressed in other half
Sometimes evident in the phenotype- cats have x-linked genes for coat color. Females that are heterozygous for coat color exhibit variagration-
patches of different colors
Pedigree Analysis
used to study the inheritance of genes in
humans.
- A series of symbols are used to represent different aspects of a pedigree.
Rules–PA
Generations in a pedigree diagram are numbered, by convention, using Roman numerals, starting with the parental generation, at the top of the diagram as generation I. The first filial generation, which
we have called F1 earlier, would be generation II in a pedigree diagram.
PA–For those traits exhibiting dominant gene action:
- Affected individuals have at least one affected parent
- The phenotype generally appears in every generation
- Two unaffected parents only have unaffected offspring
Dominant PA–Huntington’s
Huntington disease- involuntary movements and deterioration
of nervous system, eventually death. Shows up usually after 30.
Recessive gene action:
Unaffected parents can have affected offspring
affected progeny both male and female
two affected parents have only affected offspring
Recessive PA–galactosemia
galactosemia- mutation in gene that produces enzyme that breaks down lactose. Lactose is only partially broken down and galactose builds up in eyes, liver and brain. If untreated- death.
X-linked recessive
Male shows recessive phenotype more often than females - If a female is heterozygous, she has a 50% chance of son inheriting disease
X-linked recessive ex
Fragile X syndrome- in gene on X chromosome, a segment of DNA is repeated many times. This gene codes for a protein required for normal development of the brain, therefore, mental disabilities and others occur
- Colorblindness
- Hemophilia- gene that codes for a clotting factor is
mutated- bleeding is prolonged
Mitochondrial DNA
- In animals, mitochondrial DNA is transmitted by the egg and not by sperm, therefore, mitochondrial determined traits are maternally inherited (organelles and cytoplasm come from mother’s egg)
Changes in chromosome structure
Duplication, inversion, deletion, translocation
Duplication
gene segments are repeated several to 100s or thousands of times
Inversion
segment of DNA reverses itself within the gene
Deletion
loss of segment of DNA
Translocation
piece of a chromosome breaks off and attaches itself to non homologous chromosome
Aneuploidy
individuals have 1 extra or 1 less chromosome
Polyploidy
3+ of each type of chromosome
Nondisjunction
one or more pairs of chromosomes fail to separate in mitosis or meiosis, leading to cells with 1 less or 1 more chromosome
If a sperm with an extra chromosome fertilizes a normal egg,
the resulting zygote will have a trisomy (2n+1)
If a sperm with one less chromosome fertilizes a normal egg,
the resulting zygote will have a monosomy (2n-1)
Down’s Syndrome
- Occurs one in every 800-1000 births
- Trisomy of chromosome 21
- Most cases occur because of nondisjunction during gamete formation
- Occurrence of disease increases with increasing age of mother
Down’s Syndrome characteristics
Characteristics:
- Slanted eyes
- Large tongue
- Flattened facial features
- Heart defects
- Respiratory and digestive problems
- Moderate to severe mental impairment
- Abnormal skeleton- shortened body parts, loose joints
Sex Chromosome Abnormalities
Nondisjunction may occur in sex chromosomes as well- 1 in every 400
births
Turner Syndrome
Females Karyotype of XO--1 x chromosome - 98% of XO zygotes spontaneously abort - Don’t have functional ovaries - Short stature - Passive, easily intimidated by peers
XXX syndrome
female
slightly taller, slight learning disability (maybe)
normal appearance and fertility
Klinefelter Syndrome
Male XXY karyotype taller than average, slightly overweight Testes and prostate gland are smaller than average Lower testosterone production, slight estrogen production Some feminine traits-slight breasts, high pitched voice, low sperm count, low facial hair some--speech and long term memory problems sometimes testosterone can reverse the feminine effects but not the rest many are sterile
XYY condition
Male
Taller than average
slight mental impairment–most normal
