Basic Mendelian Genetics

Question 1

What is the ratio for a monohybrid cross?

Answer 1

3:1

Question 2

Explain Mendel’s law of segregation

Answer 2

One copy of each gene per parent is passed on to the progeny

Explained by gamete formation and fertilisation

The two alleles of each gene segregated during meiosis

Then, randomly, one allele from each parent will be present in the zygote

Question 3

What is the ratio for a dihybrid cross?

Answer 3

9:3:3:!

Question 4

Explain how Mendel came up the idea of independent assortment

Answer 4

Mendel performed dihybrid crosses. He discovered that the combinations of traits in the offspring of his crosses did not always match the combinations of traits in the parental organisms. From this data, he formulated the Principle of Independent Assortment.

Inheritance of one trait has no effect on the inheritance of another trait

New phenotypic combinations: recombinants: shuffling of alleles of different genes

Question 5

Explain what co-dominance is

Answer 5

The alleles in an F1 hybrid phenotype showing both of the parental traits are termed co-dominant

The offspring are essentially a mosaic of the parents

Question 6

Define Allele

Answer 6

A variant from a gene; we inherit two alleles for each gene; alleles the same (homozygous); alleles different (heterozygous)

Question 7

Define Polymorphism

Answer 7

Natural Variation within a gene; no adverse effects on the individual and occur with fairly high frequency in the general population ( greater than 1%)

Question 8

How many alleles are there in the ABO blood group?

Answer 8

3

Question 9

Which of the blood group alleles are dominant and which ones are recessive

Answer 9

O = Recessive

A and B = Dominant

Question 10

Why do alleles A and B show co-dominance?

Answer 10

For an individual who is heterozygous for these two alleles, the phenotype of both alleles is completely expressed, thus providing blood type AB

Question 11

Define Pleitrophy and give an example

Answer 11

A single gene may affect a phenotype in many ways

The allele for sickle-cell disease is an example

Question 12

Explain what happens in sickle cell anemia

Answer 12

Autosomal recessive

Functional impairment of hemoglobin beta chain; oxygen-carrying capacity of RBCs is affected

Question 13

Explain what haploinsufficiency is

Answer 13

Some phenotypes can be sensitive to the amount of functional protein produced

Heterozygote for a loss-of-function mutation that generates less than the normal amount of functional gene products may look completely different from the wild-type organism

Geneticists use the term haploinsufficiency to describe rare situations in which one wild-type allele does not provide enough of a gene product to avoid a mutant phenotype

Around 800 haploinsufficient genes in humans

Question 14

True or False genes come in alternative forms called alleles

Answer 14

True

Question 15

True or False: Two same alleles are called heterozygosity

Answer 15

False

Question 16

What is the progeny of a monohybrid cross between 2 heterozygotes?

Answer 16

3:1

Question 17

Sickle cell anemia: which of the following is correct?

A. Abnormal hemoglobin

B. Sickle-shaped red cells

C. Anaemia

D. Blocked circulation

E. Increased infection to malaria

Answer 17

E

Question 18

How can you recognize a dominant trait? Which is not correct?

A. Affected Children always have at least one affected parent

B. Two affected parents can produce unaffected children if both are heterozygotes

C. Traits always shown in males

Answer 18

C

Question 19

A genetic cross using a homozygous recessive will give a phenotype of? WHta crossed to a heterozygote?

Answer 19

1:1

Question 20

What type of children are not possible from two AB parents?

Answer 20

O

Question 21

The child is group O, mother group A., what should the father NOT be?

Answer 21

AB

Question 22

Explain what autosomal recessive is

Answer 22

Homozygotes are affected; heterozygotes are carriers.

Affected children can be the children of two unaffected carriers, particularly as a result of consanguineous matings.

All the children of two affected parents should be affected.

Rare recessive traits show a HORIZONTAL pattern of inheritance: the trait first appears among several members of one generation and is not seen in the earlier generations.

Recessive traits may show a VERTICAL pattern of inheritance if the trait is extremely common in the population.

Question 23

Give 3 examples of autosomal recessive

Answer 23

PKU

Cystic Fibrosis

Sickel Cell Anaemia

Question 24

Explain Autosomal Dominant

Answer 24

Heterozygotes and homozygous are affected.

Affected children always have at least one affected parent; at least one parent affected.

Dominant traits show a VERTICAL pattern of inheritance; the trait shows up in every generation.

Two affected parents can produce unaffected children, if both are heterozygotes.
Unaffected individuals do not produce affected children.

Question 25

Give an example of autosomal dominant

Answer 25

Huntington’s Disease

Question 26

Explain what X-linked recessive is

Answer 26

The trait appears in more males than females.

The mutation will NEVER pass from the father to the son; no offspring of an affected male are affected.

An affected male passes the X-linked mutation to all his daughters, who are thus carriers. One-half of the sons of these carrier females will inherit the defective allele and thus the trait.

The trait often skips a generation as the mutation passes from grandfather through a carrier daughter to grandson.
The trait can appear in successive generations when a sister of an affected male is a carrier. If she is, one-half of her sons will be affected.

There is always an unaffected female in the intermediate generation.
With the rare affected (homozygous) female, all her sons will be affected and all her daughters will be carriers.

Question 27

Give an example of x-linked recessive

Answer 27

Haemophilia

Question 28

Explain what x-linked dominant is

Answer 28

More females than males show the aberrant trait.

Trait seen in every generation because it is dominant.

All the daughters but none of the sons of an affected male will be affected (X-linked dominant vs autosomal dominant).

One-half of the sons and one-half of the daughters (despite the sex) of an affected female will be affected.

Question 29

Give an example of x-linked dominant

Answer 29

Hypophosphataemic rickets - (Vitamin D-independent)