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 one of the different forms of the same gene are called alleles. Alleles of the same gene occupy the same relative positions or gene loci on a pair of homologous chromosomes. Alleles of the same gene have different expressions of the same trait.

Question 4

Homologous chromosomes [2]

Answer 4

Homologous chromosomes are chromosomes which exist in pairs. Each pair have the same length, size and shape. [1] They have exactly the same sequence of genes, same centromere position and same position of gene loci. [1]

Question 5

Dominant allele

Answer 5

The allele which expresses itself in the phenotype in both homozygous and heterozygous conditions.

Question 6

Genotype

Answer 6

The genetic make-up, a combination of alleles for a particular gene of an organism.

Question 7

Phenotype

Answer 7

The expressed trait in an organism which is influenced by its genotype and the environment.

Question 8

Homozygous

Answer 8

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

Question 10

Recessive allele

Answer 10

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

Question 11

Codominant trait

Answer 11

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

Question 12

Codominance

Answer 12

Codominance is a condition where both alleles of a gene controlling a trait express themselves equally in a heterozygous genotype and in the phenotype. [1]

For instance, in blood grouping, individuals with IAIB alleles will have both alleles equally expressed and have both antigen A and antigen B on the red blood cells and have the blood group AB. [1]

Question 13

Define multiple alleles [1]

Answer 13

Alleles of the same gene or locus that has more
than two alleles for a trait. [1]

Question 14

Mutation [2]

Answer 14

Mutation is a sudden, random, spontaneous change in the structure of a gene where the sequence of nucleotides in the DNA is changed; [1] or a change in the chromosome number. [1]

Question 15

Down syndrome

Answer 15

3 copies of chromosome 21
A chromosome mutation

Question 16

Mutagens

Answer 16

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

Question 17

Variation

Answer 17

Variation refers to the differences in a particular trait between individuals of the same species.

Question 18

Differences betw discontinuous variation and continuous variation

Answer 18

1. DV: involves a few clear-cut, distinct 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 19

Causes of variation

Answer 19

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

Question 20

Natural selection

Answer 20

Natural selection refers to a process in nature which best-adapted organisms in a population with desirable genes and characteristics survive and pass on their genes to their offspring in the next generation.

Question 21

Evolution

Answer 21

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

Question 22

Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny [2]

Answer 22

Observed ratio differs from expected ratio of 3:1 because of small sample size [1] and the random nature of fertilisation from the random fusion of nuclei of gametes, which genotype of each offspring is based on probability. [1]

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 23

Describe the determination of sex in humans

Answer 23

During fertilisation, an egg will contribute a X chromosome while a sperm will contribute a X or Y chromosome. The female’s ovum can only have one copy of X chromosome, while the male’s sperm can either have a copy of X or Y chromosome. Hence the male gamete determines the sex of a child.
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.

Question 24

Sickle cell anaemia gene mutation

Answer 24

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 25

Mutagen examples

Answer 25

Radiation (UV light, gamma rays)
Mutagenic chemicals such as carcinogens eg mustard gas and tar

Question 26

Examples of environmental factors that act as forces of natural selection

Answer 26

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 27

Define natural selection and evolution. [6]

Answer 27

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 28

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

Answer 28

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 29

Incomplete dominance

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

Mutation is a change in the structure of gene or chromosome number.
Sickle cell anaemia is an example of gene mutation. [1]
The mutated gene produces haemoglobin S (HbS), which is almost the same as normal haemoglobin A (HbA) except in one amino acid. This causes a change in the three-dimensional shape of the haemoglobin molecule. HbS molecules clump together, making the cell sickle-shape. [1]
Down syndrome is an example of a mutation due to change in chromosome number. [1]
In Down syndrome, there is an extra chromosome 21 as compared to a normal person. [1]

Question 32

Explain what is meant by discontinuous variation [1]

Answer 32

Discontinuous variation refers to traits that show clear-cut/ distinct phenotypes controlled by only one or a few genes which does not have additive effect and the phenotype is not affected by environmental conditions. [1]

Question 33

Distinguish between the terms gene and allele [2]

Answer 33

A gene is a segment of DNA, containing a sequence of nucleotides that codes for a specific polypeptide while an allele is
a different form of the same gene, [1] which occupies the same gene loci on a pair of homologous chromosomes.
A gene controls the expression of a phenotypic trait but the alleles are responsible for the variations of a phenotypic trait. [1]

Question 34

When is F1 and F2 used

Answer 34

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 35

Explain, with reference to human blood groups, what is meant by codominance and multiple alleles [3]

Answer 35

Codominance: When both alleles of a gene controlling a trait are expressed equally in a heterozygous genotype. For instance, in
blood grouping, for individuals with IAIB alleles will have both alleles equally expressed and have both antigen A and antigen B on the
red blood cells and have the blood group AB. [1]

Multiple alleles: Alleles of the same gene or locus that has more than two alleles for a trait. [1] In the case of blood grouping, there
are three alleles IA, IB and IO for blood group. IA and IB alleles are codominant alleles while IO allele is a recessive allele. [1] IAIA or IAIO for blood group A, IBIB or IBIO for blood group B and IOIO for blood group O.

Question 36

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. [2]

Answer 36

Excess amino acid phenylalanine diffuses from fetal blood to maternal blood at the placenta which would be broken down by the enzyme in the mother’s blood. [1] This prevents built-up of amino acid phenylalanine in the fetus. [1]

Question 37

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

Answer 37

In co-dominance, two alleles are equally dominant in heterozygous state [1] and will exert its effects equally on the phenotype of organisms. [1]

In ABO blood group, both IA and IB alleles are codominant [1] and are dominant to IO allele which is a recessive allele. [1]

When a person have one IA allele and one IB allele in a heterozygous dominant genotype [1], both alleles are dominant and expressed equally on the phenotype codominantly, thus the person will have blood group AB. [1] An IA AB genotype person does not have any recessive lO allele to pass down to the child/ the child cannot inherit 1º allele from parent [1], hence the child will not have two recessive IO alleles expressed as blood group O phenotype.

A person with two IA alleles or one IA allele and one IO allele will have blood group A.
A person with two IB alleles or one IB allele and one IO allele will have blood group B.[1]

A person with two recessive IO alleles with a homozygous recessive genotype IO IO expressed will have blood group O.[1]

Formula:
State genotype (Heterozygous? Homozygous?) Must state genotype eg IA IO.
Alleles are recessive or dominant → one dominant to express the trait, recessive allele is masked by the dominant allele
Passing down/ inheritance of alleles from parent to child/ child from parent respectively
Expressed phenotype

Question 38

Explain whether this is a dominant or recessive condition answering technique

Answer 38

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 39

Describe how variation leads to natural selection
+ natural selection has led to the evolution of many different species of finches.

Answer 39

Spontaneous mutation(external selection pressure)/ gene mutation when mutagens are introduced, (e.g. radiation and chemicals) result in new alleles formed, contributing to the gene pool for natural selection to act on, resulting in genetic variation. [1]
Coding of different polypeptides due to change in sequence of nucleotides in segments of genes change the traits of the organisms./ Forest fires [1]
act as selection pressure. [1]
Over time, those trees that have favourable traits that confer a selective advantage survive and reproduce as a result of natural selection. [1]
They pass on the favourable genes or beneficial alleles of fire resistance to their offspring. [1]
Increasing frequency of beneficial alleles in the population. [1]

Spontaneous mutations produce new alleles and leads to genetic variation — the expression of different sizes and shapes of beak (in this case). [1]
Competition arises between different varieties of organisms due to [diff types of food sources] [1]
Natural selection selects the form [finches with beaks] that enable them to be best adapted to the environment for their selective advantage. They will survive and reproduce and [1]
pass down their favourable genes to the next generation. [1]
Other variations may die as this species becomes the predominant species.
Over time, there is an increase in allelic frequencies of genes coding for a particular beak size and shape. Finches then evolve independently, resulting in many species of finches.

Question 40

Describe how gene mutation affects polypeptide formed.

Answer 40

The sequence of nucleotides in a gene will determine the sequence of amino acids coded in a polypeptide chain. [1]
The two polypeptide chains will fold to form a three-dimensional shape lipase molecule. [1]

Question 41

Explain. why it is not possible to have a heterozygous blue-eyed person. [3]

Answer 41

Dominant allele will express itself in the phenotype and mask the effect of recessive allele. [1] AW
Blue-eyed person will have a homozygous recessive genotype, bb/ 2 recessive alleles/ 2 b alleles. [1] (must state genotype, bb)
Since both alleles are identical, not heterozygous and is homozygous[1]

Question 42

Explain the importance of meiosis, process K. [2]

Answer 42

K produces haploid cells; [1] to maintain the diploid number / restore diploid number in the zygote upon fertilisation [1] During fertilisation, nuclei of the haploid gametes fuse, forming a diploid zygote, restoring the diploid number in the zygote. This ensures that chromosome number stays constant and does not double with every successive generation.

Or
K produces gametes that are genetically different / genetic variation / genetically dissimilar; [1] To increase genetic variation [1]

Question 43

State 3 differences between the nucleus of egg and the nucleus of a zygote. [3]

Answer 43

M: haploid/23 chromosomes
N: diploid/46 chromosomes

M: has only X chromosomes
N: has either XX or XY chromosomes

M: homologous chromosomes absent
N: homologous chromosomes present

Question 44

Continuous variation

Answer 44

Refers to traits with phenotypes ranging from one extreme to the other. It is brought about by combined or additive effects of many genes and is affected by environmental conditions.