Genetics Module

Revision quiz before the first session - Answers

1. Alleles are alternative forms of a gene at the same locus
   True
   

2. When drawing a pedigree, a filled in circle is used to denote an affected female.
   True
   Females are designated by circles, males by squares. A diamond is used if the sex is unknown. A coloured in symbol denotes affected status.
   
3. The son of a woman with an autosomal dominant disorder has a ½ chance of inheriting the disorder.
   True
   
4. Males and females are affected in equal proportions in autosomal recessive inheritance.
   True
   
5. The healthy sib of a person with an autosomal recessive condition has a risk of ½ of being a carrier.
   False
   At conception a child whose parents are both carriers of an autosomal recessive condition has an equal possibility of the following genotypes – homozygous for the normal gene, heterozygous carrier, heterozygous carrier, homozygous affected. A sib of an affected person therefore has a risk of ¼ + ¼ = ½ of being a carrier at conception. However, when a sib is shown to be healthy, the risk of being an affected homozygote is removed. There remain only three possibilities – not a carrier, carrier, carrier. Therefore the risk is 2/3.
   
6. A carrier is a healthy person who is a heterozygote for a recessive trait.
   True
   Also refers to a person with a balanced chromosomal translocation
   
7. Artificial insemination by donor sperm will greatly reduce the risk of a couple with a baby with a rare autosomal recessive condition having another affected baby.
   True
   The carrier frequency for the disorder must be known. Although AI will reduce the risk, it may still be higher than expected if the autosomal recessive condition is common in the population from which the sperm donor is taken (eg the risk before carrier testing that a male in some Mediterranean populations is a carrier for thalassaemia is 1/6).
   
8. A male affected with an X-linked trait cannot transmit the disorder to his sons
   True
    A father passes on his Y chromosome to his sons. All the daughters of a male with an X-linked trait will be obligate carriers.
   
9. Females are never affected with X-linked recessive diseases.
   False
   Rarely a woman can be homozygous for an X-linked trait – eg colour blindness, when her father is affected and her mother is a carrier. If a woman has signs of an X-linked recessive disorder, then chromosome analysis should be performed to determine if she has a structural abnormality of the X chromosome, or has Turner syndrome.
   
10. In mitochondrial inheritance, affected males never pass the disease onto their children.
    True
    Mitochondria are inherited in the egg.
    
11. Human chromosome analysis by light microscopy can be performed only on peripheral blood cells.
    False
    Chromosome analysis can be performed on almost any dividing cells - commonly on skin fibroblasts, amniocytes and chorionic villus cells.
    
12. There are 45 autosomes in the normal human karyotype
    False
    There are 44 (22 pairs): an autosome is any chromosome other than the sex (X or Y) chromosomes
    
13. Reciprocal translocations are rarely inherited.
    False
    Reciprocal translocations can occur between any chromosomes, and are usually inherited. One partner in 1/20 couples with recurrent miscarriages is said to be a translocation carrier.
    
14. Robertsonian translocations are found in 25% cases of Down syndrome.
    False
     A Robertsonian translocation is the cause of Down syndrome in <5% of people with Down syndrome – when found, parents should be karyotyped to determine if the translocation is inherited, in which case other family members may be at high risk.
    
15. The primary purpose of genetic counselling is to reduce the incidence of genetic disease in the population.

False
Genetic counselling is an information giving process by which patients or relatives at risk of a genetic disorder are given information about the consequences of the disorder, the probability of developing or transmitting it, and the ways in which this may be prevented or ameliorated. It should be "non-directive", allowing families to make their own choices.