19 — inheritance

Question 1

Inheritance

Answer 1

Refers to the transmission of traits from one generation to another

Question 2

Gene

Answer 2

A gene is a sequence of DNA nucleotides that controls the formation of a single polypeptide. It s also a unit of inheritance.

Question 3

Alleles

Answer 3

The different forms of the same gene r called alleles. Alleles occupy the same relative position on a pair of homologous chromosomes.

Question 4

Homologous chromosomes

Answer 4

Homologous chromosomes
exist in pairs.

One chromosome in the pair
comes from the male parent and
the other from the female parent.

They have exactly the same
order or sequence of gene loci.

Question 5

Dominant allele

Answer 5

The expressed allele amongst 2 different alleles which gives the same phenotype in both homozygous and heterozygous conditions.

Dominant allele is represented with an upper case letter.

Question 6

Genotype

Answer 6

The combination of alleles for a particular gene is called genotype
Eg Ee EE ee

Question 7

Phenotype

Answer 7

The expressed trait or outward appearance in an organism is called phenotype. The phenotype of an organism is influenced by its genotype and the environment.

Question 8

Homozygous

Answer 8

Homozygous refers to having two identical alleles of a particular gene for a particular phenotype (e.g. DD or dd).

Question 9

Heterozygous

Answer 9

Heterozygous refers to having two different alleles of a particular gene for a particular phenotype (e.g. Dd).

Question 10

Recessive allele

Answer 10

Recessive allele is represented with
the corresponding lower case letter.

A recessive allele that only expresses itself in the phenotype in a homozygous recessive genotype as the recessive allele is masked by the dominant allele.

Question 11

Codominant trait

Answer 11

Codominant trait is when both alleles are equally expressed in the phenotype in the heterozygous condition.

Question 12

Why does the observed ratio often differ from the expected ratio?

Answer 12

Observed ratio differs from expected ratio of 3:1 because of sample size and the random nature of fertilisation

Question 13

Codominance

Answer 13

In codominance, both alleles express themselves in the heterozygote, which has a phenotype intermediate between that of its pure-bred parents. The alleles are said to be codominant.

Eg: blood group AB as IA and IB are expressed equally. Thus, they have both antigen A and B.

Question 14

Multiple alleles

Answer 14

A gene that exists in more than two different
forms is said to have multiple alleles.

Question 15

Blood groups and genotypes

Answer 15

• IA and IB are dominant over Io
• IA and IB are codominant.
  A: IAIA or IAIO
  B: IBIB or IBIO
  AB: IAIB
  O: IOIO

Question 16

Mutation

Answer 16

Mutation is a change in the sequence of a gene or in the chromosome number.

Question 17

Down syndrome

Answer 17

3 copies of chromosome 21
A chromosome mutation

Question 18

Mutagens

Answer 18

Mutagens are agents in the environment that increase the rate
of mutation in an organism.
Examples:
* Ionising radiation
* Certain chemicals

Question 19

Variation

Answer 19

Variation refers to the differences in traits between individuals of the same species.

Question 20

Discontinuous variation

Answer 20

The differences are obvious
and there are few or no
intermediate forms, clear-
cut/ distinct phenotypes

Question 21

Differences betw discontinuous variation and continuous variation

Answer 21

1. DV: involves a few clear-cut phenotypes with no intermediate forms
   CV: involves a range of phenotypes

2
DV: controlled by 1 or a few genes
CV: controlled by many genes

3. DV: genes do not show additive effect
   CV: genes show additive effect

4.
DV: relatively unaffected by env conditions
CV: greatly affected by env conditions

Examples:
CV: skin colour, height
DV: blood group, gender, eye colour

Question 22

Causes of variation and how they lead to natural selection

Answer 22

Variations in organisms may arise due to:
* crossing over and independent assortment of chromosomes and
during meiosis
* mutation in genetic material
* random fertilization of sperm and ovum

How variation leads to natural selection
1. Mutation provides new alleles to the gene pool for natural
selection to act on.
2. Genetic variation is important to help organisms adapt and survive
in changing environments due to selective advantage.
3. Competition arises due to differences in the same species, leading to differential survival and reproduction
4. Natural selection is a process in nature which results in the best-adapted organisms in a population surviving to reproduce and pass on their genes to the next generation. Nature selects varieties of organisms that are more resistant to diseases and better adapted to changes in the environment.

Question 23

Natural selection

Answer 23

Natural selection refers to a process in which organisms with desirable genes and characteristics are ‘selected’ or ‘favoured’ to survive and pass on their genes to their offspring.

Question 24

Evolution

Answer 24

The gradual change in the inheritable characteristics of a population over time is called evolution.

Question 25

Mechanism of evolution

Answer 25

1. Organisms reproduce rapidly as food supply is abundant.
2. Organisms migrate to different environments.
3. Spontaneous mutation takes place, resulting in variation in the
   organisms. Favorable traits will confer a selective advantage and
   such organisms will survive, reproduce and pass on their
   favourable genes to their offspring.
4. These organisms become the
   predominant species in their environment, resulting in evolution of the species

Question 26

Different expected ratios of offspring and their parents’ trait.

Answer 26

To obtain an expected ratio of 3:1 in the offspring generation, both parents must be heterozygous for a trait.

To obtain an expected ratio of 1:1 in the offspring generation, 1 parent must be heterozygous while the other is homozygous recessive.

Question 27

Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny

Answer 27

The genotype of every offspring is a result of random fusion of nuclei of gametes. Thus, the observed ratio and expected ratio may differ, especially when there r only a small number of offspring.

Question 28

Describe the determination of sex in humans

Answer 28

During fertilisation, an egg will contribute a X chromosome while a sperm will contribute a X or Y chromosome. Hence, sex in humans is determined by the e=sex chromosome the sperm carries.
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.

Question 29

Sickle cell anaemia gene mutation

Answer 29

1. Gene that codes for normal haemoglobin is mutated to form abnormal haemoglobin.
2. Abnormal haemoglobin causes the RBCs to be sickle shaped.
3. Sickle shaped RBCs carry less oxygen around the body
4. Homozygous recessive dies young, heterozygous survives in malaria prone areas
5. Heterozygous carry one mutated allele known as carriers.

Question 30

Causes of mutation

Answer 30

Radiation (UV light, gamma rays)
Chemicals such as carcinogens

Question 31

Examples of environmental factors that act as forces of natural selection

Answer 31

1. Competition for food
2. Competition for territory
3. Climate
4. Types of predators
5. Availability of mates
6. Process by which present complex forms of living organisms have arisen from simpler ancestral forms is known as evolution
7. To ensure that the organisms best adapted to the environment will survive to maturity and reproduce to pass down their genes.

Question 32

Role of natural selection as a possible mechanism for evolution

Answer 32

1. Organisms reproduce rapidly as food supply is abundant
2. Organisms migrate to different environments
3. Spontaneous mutation takes place, resulting in genetic variation in the organisms. Favourable traits will confer a selective advantage and such organisms will survive, reproduce and pass on their favourable genes to their offspring.
4. All species tend to produce more offspring Han the environment can support
5. These organisms become the predominant species in their environment. Offspring of survivors will inherit their advantageous characteristics and remain well adapted to environment.

Question 33

Define natural selection and evolution. [6]

Answer 33

Organisms having desirable traits adaptable to the environment will survive [1] and reproduce to pass on their genes with the same desirable traits [1], whereas organism not having the desirable traits will not be able to survive [1] and reproduce, thus they will perish overtime. [1].
After many generations [1], the survival of the fittest will result in the gradual shift in the phenotype and genotype of the population [1] leading to evolution of the species.

Question 34

Explain why it is impossible for parents of AB and O blood group to have a biological child of blood group O. [3]

Answer 34

O blood group is homozygous recessive IO IO [1] which requires two recessive alleles to be expressed. The child must inherit one IO allele from each parent [1] but the AB blood group parent does not have any IO allele to pass down while the O blood group parent can pass down one IO allele [1], thus the child will not have two recessive alleles IOIO to be blood group O.