Topic 1 Heredity: Human Genetic Defects

Question 1

• There are a number of genetic defects that can be caused by mutations, environmental factors, or damage to chromosomes. The section summarizes these defects and subsequent genetic disorders.

Answer 1

Note

Question 2

• Single nucleotide change causing either substitution, insertion, or deletion—the latter two can cause a frameshift mutation
• A transition mutation involves conversion of a purine to purine or pyrimidine to pyrimidine. A transversion mutation involves conversion of a purine to pyrimidine or vice versa.

Answer 2

1. Point Mutation

Question 3

• A genome with extra or missing chromosomes, often caused by nondisjunction
  i. One example of aneuploidy is Down syndrome, or trisomy 21
  ii. Turner syndrome is a genetic condition in which a female is either completely missing, or partly missing, an X chromosome, leading to the genotype XO. Turner syndrome also occurs as a result of nondisjunction, and the main genetic defect is the offspring is born with physical abnormalities.
  iii. A third genetic condition is Klinefelter’s Syndrome (XXY), in which a male is born with an extra X chromosome

Answer 3

2. Aneuploidy

Question 4

• Chromosome segments are changed
  i. Duplications - chromosome segments are repeated on the same chromosome, which can occur from unequal crossing over
  ii. Inversions - chromosome segments are rearranged in reverse orientation
  iii. Translocations - one segment of a chromosome is moved to another chromosome.
  a. Can be reciprocal (two non- homologous chromosomes swap segments) or Robertsonian (one chromosome from a homologous pair becomes attached to another chromosome from a different pair). For example, an extra chromosome 21 attached to chromosome 14 can cause Down syndrome as well, due to the tripled 21 chromosome segment.

Answer 4

3. Chromosomal Aberration

Question 5

• There is no gain or loss of genetic information in a reciprocal translocation, while there is a loss of genetic information in a Robertsonian translocation.

Answer 5

Note

Question 6

• Spontaneous or induced breakage of a chromosomal segment via mutagenic agents or X-rays

Answer 6

4. Chromosomal Breakage

Question 7

• Include cosmic rays, X-rays, UV rays, radioactivity, chemical compounds including colchicine and mustard gas that can cause genetic mutations. Mutagenic agents are generally also carcinogenic

Answer 7

5. Mutagenic Agents

Question 8

• This following list includes all of the genetic disorders that are likely to appear on the DAT

Answer 8

6. Genetic Disorders

Question 9

a. Autosomal recessive conditions
b. Autosomal Dominant conditions
c. Sec-linked recessive conditions
d. Chromosomal Disorders

Answer 9

Types of Genetic disorders

Question 10

a. Phenylketonuria (PKU) - inability to produce the proper enzyme for phenylalanine breakdown, causing degradation product phenylpyruvic acid to accumulate
b. Cystic fibrosis - fluid buildup in respiratory tracts
c. Tay-sachs - lysosome defect in which cells can’t breakdown lipids for normal brain function

Answer 10

a. Autosomal recessive conditions

Question 11

a. Huntington’s disease – nervous system degeneration
b. Achondroplasia - causes dwarfism
c. Hypercholesterolemia – excess cholesterol in blood that progresses into heart disease

Answer 11

b. Autosomal Dominant Conditions

Question 12

a. Hemophilia - sex-linked recessive genetic condition causing abnormal blood clotting
b. Color blindness – primarily observed in males
c. Duchenne’s Muscular Dystrophy - progressive loss of muscle

Answer 12

c. Sex-linked recessive Conditions

Question 13

• Changes to the number or structure of chromosome
  a. Down’s Syndrome - trisomy 21
  b. Turner’s Syndrome - deletion of X chromosome → XO genotype
  c. Klinefelter’s Syndrome - extra X chromosome → XXY genotype
  d. Cri du Chat - deletion on chromosome 5

Answer 13

d. Chromosomal disorders

Question 14

• A fetus can be tested for genetic disorders via amniocentesis or chorionic villus sampling (CVS)

Answer 14

Note