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 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.

Question 35

Incomplete dominance

Answer 35

When two different alleles are expressed resulting in phenotype being an intermediate between the two.
Eg blood group AB

Question 36

Suggest one factor which increases the rate of mutation [1]

Answer 36

Mutagenic radiation like UV light, alpha, beta and gamma radiation
Chemical mutagens such as mustard gas, formaldehyde and tar

Question 37

Using named examples, explain what is meant by a mutation [4]

Answer 37

A mutation is a change in gene sequence or chromosome number. For example, sickle-cell anaemia in humans is caused by a gene mutation. The gene controlling haemoglobin production is mutated and produces abnormal haemoglobin which clumps tgt. The RBCs r sickle-cell shaped as a result and cannot transport oxygen efficiently.
Down’s Syndrome is an example of chromosomal mutation. Patients with DS have 47 chromosomes instead of 46, as they receive 1 more chromosome 21 during fertilisation. They have characteristic facial features and face mental and physical difficulties.

Question 38

Explain what is meant by discontinuous variation [1]

Answer 38

Clearly defined differences in a trait with no or few intermediate forms.

Question 39

Distinguish between the terms gene and allele

Answer 39

A gene is a segment of DNA consisting of a specific sequence of nucleotides that code for a functional polypeptide. An allele refers to different forms of the same gene.

Question 40

Suggest how radiation and chemicals could influence the evolution of a population [3

Answer 40

Radiation and chemicals may randomly alter the DNA sequence or number of chromosomes of an organism, leading to the gaining of a new allele that results in a trait that confers a selective advantage for the organism over the others in the population. This organism will have a better chance to survive and reproduce as compared to those without mutation, producing more offspring that confers the selective advantage. Over many generational frequency of individuals with mutated allele conferring the advantageous trait will increase, leading to the evolution of the population.

Question 41

When is F1 and F2 used

Answer 41

They’re used when parents (at least 1) are heterozygous. If both are homozygous, offspring uses F1 and parent just use parental genotype etc.

Question 42

Disease (recessive) in genetic diagram

Answer 42

When disease is recessive: d

DD -> heathy
Dd -> carrier
Dd-> unhealthy/name of disease

Question 43

Explain, with reference to human blood groups, what is meant by codominance and multiple alleles

Answer 43

Codominance: occurs when 2 alleles r equally expressed in the heterozygous condition. IA and IB alleles are codominant and an individual with both alleles will have the AB blood group

Multiple alleles: occurs when there are 3 or more different alleles for a particular trait. For example, the human blood group has 3 multiple alleles: IO, IA and IB.

Question 44

A baby with PKU (a disease) is growing inside the uterus of a mother who does not have PKU. It is developing normally. Suggest why the baby in the trus does not show any symptoms of PKU.

Answer 44

The enzyme needed to break down phenylalanine is able to pass thr the placenta from the mother’s blood to fetus’ bloodstream, allowing phenylalanine to be broken down and thus no symptoms of PKU in the baby displayed while n the uterus.

Question 45

With reference to the inheritance of ABO blood group phenotypes, explain what is meant by codominance. [6]

Answer 45

Co-dominance occurs when 2 alleles controlling a trait both express themselves in the phenotype. Neither allele is completely dominant over the other. The 4 blood groups A,B,AB, and O are determined by alleles IA, IB, and IO. Alleles IA and IB are co-dominant while allele IO is recessive. Thus allele IP does not express itself in homozygous condition. Both alleles IA and IB expresses themselves in both homozygous and heterozygous conditions and thus are codominant to each other. Thus, individuals with blood group IAIB will have blood group AB.

Question 46

Explain why there is a discrepancy between the observed and expected offspring ratio. [2]

Answer 46

As fertilisation is a random process, the genotype of each offspring is based on chance/probability.
The smaller the sample size of offspring, the greater the discrepancy would be.

Question 47

Explain whether this is a dominant or recessive condition answering technique

Answer 47

Dominant/recessive. As x have the disorder/does not but parents …
x must be homozygous/heterozygous recessive/dominant and inherited one recessive/dominant allele from parent 1 and from parent 2.

Question 48

Explain whether the male or female gamete determines the sex of a child. [3]

Answer 48

In females, each nucleus has two X chromosomes, while males have one X and one Y chromosome.

During meiosis, sex chromosomes segregate into different gametes.

The female’s ovum can only have an X chromosome, while the male’s sperm can either have an X or Y chromosome,

Hence the male gamete that determines the sex of a child.

Question 49

A population of normal rabbits migrated to a dense forest. Explain how they could evolve to have small body size and dark fur. [5]

Answer 49

In the forest, the presence of predators acts a selection pressure.
As there is variation between rabbits, some of them had smaller body sizes, and some of the had darker fur.

These were favourable traits, as smaller body size allowed them to evade predators more easily, while darker fur allowed them to camouflage in the forest to avoid being spotted, giving them a selective advantage.
As a result, these individuals survived and reproduced more, passing on their favourable alleles to their offspring.
Over a long period of time, accumulation of new genes by mutation and natural selection resulted in the evolution of the species to have small bodies and dark fur.

Question 50

Evolution by natural selection answering technique

Answer 50

V: Variation exists between individuals
A: selective Advantage
S: Survive and reproduce, passing favourable alleles to offspring
T: over Time, more and more of the population has the favourable trait

Question 51

In one part of the world, eucalyptus trees have evolved to resist fires. Explain how these trees have evolved to resist fires. [5]

Answer 51

A few trees acquired continuous mutation that enables the trees to become more fire resistant. The climate in that region became drier and forest fires became more cool. Trees that r not fire resistant are burnt and are unable to disperse seed.s the few trees with the mutation that confers the fire resistance will have a selective advantage thus survive the fires. These trees can reproduce and disperse seeds containing the favourable fire resistance genes.