Lecture 5: Inheritance and genetic diseases

Question 1

What is asexual reproduction?

Answer 1

The production of clones without exchanging genetic information with another organism through sex.

Question 2

What is sexual reproduction?

Answer 2

The production of new living organisms by combining genetic information from two individuals gametes

Question 3

What is the benefit of sexual reproduction?

Answer 3

1. Results in greater genetic variation
2. Allows for the beneficial combination of genes
3. Facilitates removal of deleterious combinations of genes.

Question 4

How is genetic variation generated?

Answer 4

1. Mutation (errors in DNA replication and repair)
2. Diploid cells (inherit one set of chromosomes from each parent)
3. Homologous recombination (HR) between chromosomes during meiosis

Question 5

Describe the process of meiosis (reductional division) in gametes (sex cells)

Answer 5

1. Normal meiosis 1 is similar to mitosis. Crossing over of chromosomes in prophase 1 results in exchange of genetic material (homologous recombination). 2. Separation of homologs at anaphase of meiosis 1.
2. Chromosomes condense again, following a brief interphase (interkinesis) in which DNA does not replicate.
3. Separation of sister chromatids at equatorial plate during anaphase of meiosis 2.
4. Chromosomes gather into nuclei in telophase and cells divide.
5. Production of 4 non-identical cells with a haploid number of chromosomes.

Question 6

Describe the process of homologous recombination

Answer 6

Caused by crossover events where highly similar DNA sequence binds to complementary sequence on homologous chromosome and there is the potential for a transfer of genetic material between the chromosomes. Random process driven by enzymes.

Question 7

What causes the genetic variation during gamete formation?

Answer 7

1. Independent assortment of chromosomes

2. Homologous recombination (HR)

Question 8

What is non-disjunction in meiosis?

Answer 8

Failure of homologous chromosomes or sister chromatids to separate properly during cell division which causes early miscarriages or birth defects.

Question 9

What increases the risk of non-disjunction?

Answer 9

Risk of non-disjunction increases with maternal age and so age is an important element of pre-natal testing.

Question 10

Name 3 disorders caused by non-disjunction

Answer 10

1. Trisomy 21: Down’s Syndrome
2. Monosomy: Turner Syndrome
3. XXY: Klinefelter syndrome

Question 11

How are genetic diseases passed down through the generations?

Answer 11

1. Autosomal Dominant
2. Autosomal Recessive
3. X-Linked

Question 12

Trisomy 21 (Down’s Syndrome)?

Answer 12

• Caused by 3 copies of chromosome 21
• Non-specific effects such as intellectual disability, broad flat face, slanting eyes, congenital heart disease, intestinal blockage and enlarged colon.

Question 13

Monosomy Turner Syndrome?

Answer 13

• Affects females
• Leads to pubertal failure and infertility and can result in babies born with neck webbing, heart defects and horseshoe kidneys
• Triggers X-inactivation

Question 14

Klinefelter syndrome?

Answer 14

• Affects males

* Leads to reduced fertility and lower IQ; may go undiagnosed.

Question 15

Describe autosomal dominant

Answer 15

Prominent allele; a single copy of a disease-associated mutation is enough to cause the disease if the gene is dominant.

Question 16

Describe autosomal recessive

Answer 16

Non-prominent allele which requires two copies in order to be expressed. To have an autosomal recessive disorder, you inherit two mutated genes, one from each parent.

Question 17

Describe X-Linked inheritance

Answer 17

Pattern of inheritance for a genetic condition that occurs when a copy of a gene located on the X chromosome has a genetic variant.

Question 18

What is the genotype?

Answer 18

The set of genes carried by an individual

Question 19

What is the phenotype?

Answer 19

The observable characteristic of an organism

Question 20

What is an allele?

Answer 20

An alternative form/different version of a gene

Question 21

Describe homozygous

Answer 21

Two identical alleles

Question 22

Describe heterozygous

Answer 22

Two different alleles

Question 23

Name an X-linked disorder

Answer 23

Haemophilia A: x-linked recessive disease caused by deletions or inversions in F8 gene which results in a loss of function and a lack of blood clotting factor VIII.

Question 24

What is Patau’s Syndrome?

Answer 24

Trisomy 13

Question 25

What is Edward’s Syndrome?

Answer 25

Trisomy 18