Human Genetics 4 & 5

Question 1

Describe crossing over

Answer 1

• During Prophase 1: crossing over of non-sister chromatids
• Exchange genetic material
• Pairing structure of homologous chromosome:
• Tetrad (4 chromatids) = synapsis- bivalent

Question 2

Define chiasmata

Answer 2

points of crossing over

Question 3

describe independent assortment

Answer 3

in anaphase 1 and 2, either homologous chromosome or either cromatid drawn to different poles at random

Question 4

which gametes are motile

Answer 4

sperm

Question 5

what are the names of the male and female sex cells

Answer 5

o male = spermatozoa

female = ova

Question 6

what are chromosomes ordered by in karyotypes

Answer 6

1. length and location

of centromer

Question 7

female male karyotypes

Answer 7

Female karyotype: 46, XX

Male karyotype: 46, XY

Question 8

cause is the cause of numerical chromosome aberrations

Answer 8

: failure of segregation= non disjunction (in meiosis I or II)

Question 9

what occurs at normal segregation

Answer 9

1. Homologous chromosomes get ‘separated’ = drawn to different poles in anaphase I
2. Sister chromatids get separated= drawn to different poles in anaphase II

Question 10

define aneuploidy

Answer 10

abnormal number of individual chromosomes in a cell

Question 11

what trisomy and what ones are possible for humans

Answer 11

extra chromo

13 18 21

Question 12

what are four non disjunction syndrome for sex chromosomes

Answer 12

• Monosomy X (Turner Syndrome)
• Trisomy X- Triple X syndrome
• Klinefelter Syndrome
• 47 XYY males

Question 13

describe features of Turner syndrome

Answer 13

• Karyotype 45X
• Only viable monosomy (missing one chromosome) in humans
• Genetically female

Question 14

describe features of triple x syndrome

Answer 14

• 47 XXX female
• ## Healthy and fertilePhenotypically normal

Question 15

describe features of klinefelter

Answer 15

• 47 XXY
• Breast enlargement, other feminine characteristics
• Unusually small testes
- male

Question 16

describe features of 47 XYY males

Answer 16

• Nondisjunction in second meiotic division (of father)

* Phenotypically normal but taller than average

Question 17

describe sex determination in mammal embryos

Answer 17

• Humans: f/m embryos phenotypically identical till 7th week

Sexual differentiation when sex determining region on y chromosome directs bipotential gonad to turn into testes not ovaries
• Differentiation and dimorphism arises from circulation of sex steroid
• Absence of Y chromosome the embryo will develop female characteristics

Question 18

describe the compensation mechanism for x chromosome inactivatation

Answer 18

1. Inactivation of one X chromosome (16 days post fertilization by packaging heterochromatin
2. Becomes silenced = transcriptionally inactive)
3. Inactive= Barr bodies
4. All descendent cells keep same inactive X

Question 19

what are 3 non disjunction syndrome for autosomes chromosomes

Answer 19

Patau syndrome (c13)

Edwards syndrome (trisomy 18)

• Down Syndrome (trisomy 21)

Question 20

describe Patau syndrome (c13)

Answer 20

• Karyotype 47, +13
• Trisomy of 13: extra copy
• Severe disabilities

Question 21

describe Edwards syndrome (trisomy 18)

Answer 21

• Karyotype: 47, +18
• Trisomy 18
• Lethal

Question 22

describe Down Syndrome (trisomy 21)

Answer 22

Karyotype: 47, 21+

• Altered phenotype

Question 23

describe the age effect of trisomy 21

Answer 23

0. Highly correlated with age of mother
1. Risk associated with delay in competition of prophase I in female gametes
2. Paternal age important too (1/4 cases originate from paternal non disjunction)

Question 24

describe polyploidy

Answer 24

Presence of two or more complete sets of chromosomes (tetraploids 4n)
• Often due to duplication of the genome without subsequent mitosis/cell division

Question 25

what can cause polyploidy in humans

Answer 25

0. Dispermic fertilization of the egg

1. Failure to extrude one polar body from the oocyte

Question 26

what are symptoms of inborn errors of metabolism due to

Answer 26

• Accumulation of substrate (e.g. alkaptonuria)
• Deficiency of product (e.g. albinism)
• Disturbance of related pathways (e.g. PKU)