Inheritance

Question 1

Explain the role of the nucleus in a cell and describe the relationship between chromosomes and genes.

Answer 1

The nucleus controls the cell’s activities and contains chromosomes made of DNA. Genes are sections of DNA found on chromosomes, and they code for specific proteins that determine the cell’s functions and inherited traits.

Question 2

Define the term “genome.”

Answer 2

The genome is the entire DNA of an organism.

Question 3

Explain the relationship between a gene and a molecule of DNA

Answer 3

A gene is a section of a DNA molecule that codes for a specific protein.

Question 4

Describe the structure of a DNA molecule, including the arrangement of its strands and the pairing of its bases.

Answer 4

DNA consists of two strands coiled into a double helix. The strands are linked by base pairs: adenine (A) with thymine (T), and cytosine (C) with guanine (G).

Question 5

Describe the structure of an RNA molecule and explain how it differs from a DNA molecule in terms of its bases.

Answer 5

RNA is single-stranded and contains the sugar ribose instead of deoxyribose and uracil (U) instead of thymine (T), while DNA is double-stranded and contains deoxyribose instead of ribose and thymine (T) instead of uracil (U).

Question 6

Describe the stages of protein synthesis, including transcription and translation, and the roles of mRNA, ribosomes, tRNA, codons, and anticodons.

Answer 6

In transcription, DNA is used to make mRNA in the nucleus. The mRNA then moves to the ribosome. In translation, the ribosome reads the mRNA codons. tRNA brings amino acids to the ribosome, matching its anticodon with the mRNA codon. The amino acids are joined together to form a protein.

Question 7

Explain what alleles are and describe how they give rise to differences in inherited characteristics.

Answer 7

Alleles are alternative forms of the same gene. Different alleles can lead to variations in inherited characteristics. For example, one allele might code for brown eyes, while another codes for blue eyes. The combination of alleles an individual inherits from their parents determines their traits.

Question 8

Explain the difference between the diploid and haploid number of chromosomes in human cells, including their values.

Answer 8

In human cells, the diploid number of chromosomes is 46, meaning there are two sets of chromosomes, one from each parent. The haploid number is 23, which represents a single set of chromosomes found in gametes (sperm and egg cells)

Question 9

Explain what a mutation is and describe how it can be inherited.

Answer 9

A mutation is a rare, random change in genetic material (DNA). If a mutation occurs in a gamete (sperm or egg cell), it can be passed on to offspring and inherited by the next generation.

Question 10

Explain how a change in DNA can affect the phenotype by altering the sequence of amino acids in a protein.

Answer 10

A change in DNA, called a mutation, can alter the sequence of bases in a gene. This change may result in a different sequence of amino acids in the protein that the gene codes for. Since the structure and function of a protein depend on its amino acid sequence, this can affect the organism’s phenotype, potentially changing its physical or functional traits.

Question 11

Describe how genetic mutations can affect the phenotype, and explain why most mutations have no effect.

Answer 11

Most genetic mutations have no effect on the phenotype because they occur in non-coding regions of DNA or do not change the function of the protein. Some mutations may cause a small effect if they slightly alter the protein, but the organism can still function normally. Rarely, a mutation can have a significant effect, leading to major changes in the phenotype if it alters a critical protein or cell function.

Question 12

Explain how the incidence of mutations can be increased, giving examples of factors that can cause mutations.

Answer 12

The incidence of mutations can be increased by exposure to mutagens like ionising radiation, such as gamma rays, x-rays, and ultraviolet (UV) rays. Some chemical mutagens, like those found in tobacco, mustard gas and nitrous oxide, can also raise the rate of mutations. These factors can damage the DNA, leading to changes in the genetic material.

Question 13

Explain how the division of a diploid cell by mitosis produces two genetically identical cells, including the number of chromosomes in each cell.

Answer 13

Mitosis is a type of cell division that produces two genetically identical diploid cells. During mitosis, the DNA is copied, and the chromosomes are evenly distributed to each new cell. As a result, both daughter cells have the same number of chromosomes as the original diploid cell, which in humans is 46.

Question 14

Describe the roles of mitosis in growth, repair, cloning, and asexual reproduction.

Answer 14

Mitosis is essential for growth as it allows organisms to increase the number of cells. It also plays a role in repairing damaged tissues by replacing cells. In cloning, mitosis ensures that genetically identical copies are produced. In asexual reproduction, mitosis produces offspring that are genetically identical to the parent.

Question 15

Explain how meiosis leads to the formation of four genetically different haploid gametes, including the number of chromosomes in each cell.

Answer 15

Meiosis is a type of cell division that produces four genetically different haploid gametes. During meiosis, a diploid cell undergoes two rounds of division, and the number of chromosomes is halved. Each resulting gamete has half the number of chromosomes as the original diploid cell (23 in humans), and the combination of chromosomes in each gamete is different, resulting in genetic variation.

Question 16

Explain how random fertilisation leads to genetic variation in offspring.

Answer 16

Random fertilisation occurs when any sperm cell can fuse with any egg cell, each of which carries a unique combination of alleles due to meiosis. This random combination of genetic material from both parents increases genetic variation in the offspring, as each fertilised egg will have a different mix of parental genes.

Question 17

Explain how variation within a species can be caused by genetic factors, environmental factors, or a combination of both, giving examples.

Answer 17

Variation within a species can be caused by genetic factors, such as differences in alleles inherited from parents, which result in traits like eye color or blood type. Environmental factors, like nutrition or climate, can also influence traits such as body weight or plant growth. Often, both genetic and environmental factors interact, such as height, which is influenced by genes but can be affected by nutrition during childhood.

Question 18

Define the following genetic terms: dominant, recessive, homozygous, heterozygous, phenotype, and genotype.

Answer 18

-Dominant: An allele of a gene that is expressed in the heterozygote
-Recessive: An allele that is not expressed in the phenotype when a dominant allele of a gene is present in the heterozygote
-Homozygous: Having two identical alleles for a particular gene. e.g. AA or aa
-Heterozygous: Having two different alleles for a particular gene. e.g. Aa
-Phenotype: The physical appearance or characteristics of an organism.
-Genotype: The genetic makeup of an organism, including its alleles.

Question 19

Describe the pattern of monohybrid inheritance for a trait using a genetic diagram. Use a cross between a homozygous dominant individual and a homozygous recessive individual as an example.

Answer 19

Monohybrid inheritance refers to the inheritance of a single trait controlled by one gene with two alleles. For example, if we cross a homozygous dominant individual (BB) with a homozygous recessive individual (bb), All offspring will inherit one dominant allele (B) and one recessive allele (b), resulting in the heterozygous genotype (Bb) and expressing the dominant trait in the phenotype.

Question 20

Define codominance and give an example of a trait that shows codominance in humans

Answer 20

Codominance occurs when both alleles in a gene pair are fully expressed in the phenotype, and neither allele is dominant over the other. An example of codominance in humans is the ABO blood group system. Individuals with the genotype IAIB have both A and B antigens on their red blood cells, showing both traits equally.

Question 21

Explain how the sex of a person is determined by their chromosomes.

Answer 21

The sex of a person is determined by one pair of sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). During reproduction, the mother passes on one of her X chromosomes, while the father can pass on either an X or a Y chromosome. If the sperm carrying an X chromosome fertilises the egg, the offspring will be female (XX). If the sperm carrying a Y chromosome fertilises the egg, the offspring will be male (XY).

Question 22

Explain how most phenotypic features are the result of polygenic inheritance, rather than being controlled by a single gene. Give an example of a phenotypic feature that shows polygenic inheritance.

Answer 22

Polygenic inheritance occurs when multiple genes influence a single phenotypic feature. Unlike single-gene inheritance, where one gene controls a trait, polygenic traits are controlled by several genes, each contributing a small effect. This leads to a range of variation in the phenotype.

An example of polygenic inheritance is human skin color. It is controlled by multiple genes, each with several alleles, leading to a wide range of skin tones. The more dominant alleles a person has, the darker their skin will be. This explains why skin color shows continuous variation rather than distinct categories

Question 23

Explain Darwin’s theory of evolution by natural selection. Use an example to illustrate your answer.

Answer 23

Darwin’s theory of evolution by natural selection suggests that organisms better adapted to their environment are more likely to survive and reproduce. This process occurs through the following steps:

1. Variation: In any population, individuals show variation in their traits due to genetic differences.
2. Competition: There is competition for resources like food, shelter, and mates, leading to a struggle for survival.
3. Survival of the fittest: Individuals with traits that give them an advantage are more likely to survive and reproduce. These advantageous traits are passed on to their offspring.
4. Inheritance: Over many generations, the traits that improve survival become more common in the population, leading to evolutionary changes.

For example, in a population of giraffes, some have slightly longer necks due to genetic variation. In times of food scarcity, giraffes with longer necks can reach higher branches and access more food. These giraffes are more likely to survive and pass on the genes for longer necks to their offspring. Over generations, the population evolves to have predominantly long-necked giraffes.

Question 24

Explain how resistance to antibiotics can increase in bacterial populations, and describe how this makes infections more difficult to control.

Answer 24

Resistance to antibiotics in bacterial populations increases through natural selection. Here’s how the process works:

1. Variation: In a population of bacteria, some bacteria may have a mutation that makes them resistant to a particular antibiotic.
2. Exposure to Antibiotics: When antibiotics are used, they kill most of the non-resistant bacteria, but the resistant bacteria survive.
3. Reproduction: The resistant bacteria reproduce and pass on the genes for antibiotic resistance to their offspring.
4. Spread of Resistance: Over time, the population of resistant bacteria increases, making the antibiotic less effective at controlling the infection.

This makes infections more difficult to control because common antibiotics no longer work, leading to the need for stronger or alternative treatments. In some cases, multi-drug-resistant bacteria can develop, which are harder to treat and require more complex and costly medical interventions.