Human Genetics

Question 1

Linked genes

Answer 1

two loci that lie close to one another on the same chromosome

Question 2

Mendel’s law of Independent assortment is not always accurate–why?

Answer 2

Crossovers and Recombinations

Question 3

Mendel’s law of Independent assortment

Answer 3

states that gene pairs

assort independently of other gene pairs

Question 4

Thomas Morgan’s experiment

Answer 4

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

Question 5

recombination–

Answer 5

crossing over to make
combinations that weren’t originally
there

Question 6

Calculating the Frequency of Crossing over

Answer 6

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.

Question 7

when is crossing over more likely to occur? how is this info useful?

Answer 7

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

Question 8

The actual amount of crossovers is more than we see because

Answer 8

there can be crossing over between the two loci which does not change the combination of genes

Question 9

X- linked genes

Answer 9

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

Question 10

holandric

Answer 10

Genes on the Y chromosome but not the X chromosome (330)
ex–hairy earlobes, SRY gene–sex
determining region of y chromosome l

Question 11

female receives (x,y)

Answer 11

x from mother and x from father

Question 12

male receives (x,y)

Answer 12

x from mother and y from father

Question 13

hemizygous

Answer 13

(neither homozygous nor heterozygous- only has one copy of the X)- will always express whatever allele is present on x chromosome.

male.

Question 14

Dosage compensation

Answer 14

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

Question 15

Barr body

Answer 15

inactivated x chromosome; visible as dark spot on edge of nucleus

Question 16

Dosage compensation–heterozygous females

Answer 16

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

Question 17

Pedigree Analysis

Answer 17

used to study the inheritance of genes in
humans.
- A series of symbols are used to represent different aspects of a pedigree.

Question 18

Rules–PA

Answer 18

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.

Question 19

PA–For those traits exhibiting dominant gene action:

Answer 19

• Affected individuals have at least one affected parent
• The phenotype generally appears in every generation
• Two unaffected parents only have unaffected offspring

Question 20

Dominant PA–Huntington’s

Answer 20

Huntington disease- involuntary movements and deterioration

of nervous system, eventually death. Shows up usually after 30.

Question 21

Recessive gene action:

Answer 21

Unaffected parents can have affected offspring
affected progeny both male and female
two affected parents have only affected offspring

Question 22

Recessive PA–galactosemia

Answer 22

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.

Question 23

X-linked recessive

Answer 23

Male shows recessive phenotype
more often than females
- If a female is heterozygous, she has a
50% chance of son inheriting
disease

Question 24

X-linked recessive ex

Answer 24

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

Question 25

Mitochondrial DNA

Answer 25

• 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)

Question 26

Changes in chromosome structure

Answer 26

Duplication, inversion, deletion, translocation

Question 27

Duplication

Answer 27

gene segments are repeated several to 100s or thousands of times

Question 28

Inversion

Answer 28

segment of DNA reverses itself within the gene

Question 29

Deletion

Answer 29

loss of segment of DNA

Question 30

Translocation

Answer 30

piece of a chromosome breaks off and attaches itself to non homologous chromosome

Question 31

Aneuploidy

Answer 31

individuals have 1 extra or 1 less chromosome

Question 32

Polyploidy

Answer 32

3+ of each type of chromosome

Question 33

Nondisjunction

Answer 33

one or more pairs of chromosomes fail to separate in mitosis or meiosis, leading to cells with 1 less or 1 more chromosome

Question 34

If a sperm with an extra chromosome fertilizes a normal egg,

Answer 34

the resulting zygote will have a trisomy (2n+1)

Question 35

If a sperm with one less chromosome fertilizes a normal egg,

Answer 35

the resulting zygote will have a monosomy (2n-1)

Question 36

Down’s Syndrome

Answer 36

• 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

Question 37

Down’s Syndrome characteristics

Answer 37

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

Question 38

Sex Chromosome Abnormalities

Answer 38

Nondisjunction may occur in sex chromosomes as well- 1 in every 400
births

Question 39

Turner Syndrome

Answer 39

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

Question 40

XXX syndrome

Answer 40

female
slightly taller, slight learning disability (maybe)
normal appearance and fertility

Question 41

Klinefelter Syndrome

Answer 41

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

Question 42

XYY condition

Answer 42

Male
Taller than average
slight mental impairment–most normal