Chapter 15 - Inheritance

Question 1

What are the two principles proposed by Gregor Mendel when he studied the reproduction in pea plants ?

Answer 1

1. The various hereditary characteristics are controlled by factors (that we now call genes) and these occur in pairs.
2. During the formation of the gametes (in humans, the eggs and the sperm), the pairs of factors separate. Each gamete receives only one set of factors, or genes. Gametes unite at fertilisation, allowing different combinations of genes to come together. This became known as the principle of independent assortment (or sometimes the law of independent assortment).

Question 2

Define Gene.

Answer 2

The factor that determines an inherited characteristic; located in the chromosomes; an individual normally has only two copies of each gene.

Question 3

Define Chromosomes.

Answer 3

Rod-like structures in the cell nucleus that carry the hereditary information.

Question 4

What was the problem with Mendel’s principle of independent assortment?

Answer 4

• It is only true if the gene for each trait under consideration is located in a separate pair of chromosomes.
• However, Mendel did not know of the existence of chromosomes.
• All the results he achieved were from observations of breeding experiments.

Question 5

Chromosomes in Humans.

Answer 5

• Humans have 23 pairs of chromosomes and many, many more characteristics, each determined by one or more pairs of genes.
• It would thus be impossible for each pair of genes to be located in a separate pair of chromosomes; each pair of chromosomes must contain many more than on pair of genes.
• In fact, a human cell has about 20 000 to 25 000 pairs of genes located in the 23 pairs of chromosomes.

Question 6

What did the American graduate student, Walter Sutton discover?

Answer 6

• He was able to link the work of Mendel to that of the cell biologists.
• His observations of the behaviour of chromosomes during the formation of gametes led Sutton to the hypothesis that the hereditary factors, or genes, were located in the chromosomes.
• This important hypothesis, contained in a research paper he published in 1903, led to the chromosome theory of heredity.
• However, Sutton did not use the term ‘gene’: it was first introduced as ‘gen’ by German biologist Wilhelm W. Johannsen in 1909.

Question 7

From his investigations, Sutton discovered a number of parallels between the way chromosomes behaved and the way genes were thought to act. List them.

Answer 7

• Chromosomes and genes both occurred in pairs in the zygote and in all somatic cells, or body cells.
• Chromosomes and genes both segregated during meiosis, and only one member of each pair normally entered a gamete ( a sperm or an egg cell).
• Chromosomes and genes both maintained their individuality during segregation, and each pair segregated independently of every other pair.

Question 8

What is Meiosis?

Answer 8

Meiosis is the form of cell division in which four daughter cells are produced that each have half the normal chromosome number.

Question 9

Explain the process of Meiosis.

Answer 9

1. Diploid cell before meiosis.
   - This hypothetical chromosome number of 4. Note the alleles on the chromosomes.
2. First Meiotic Division
   - The homologous chromosomes have separated. Note the alleles present in the cell at this stage.
3. Second meiotic division
   - The chromatids have separated and the division has produced 4 haploid gametes.

Question 10

How did Sutton come to the conclusion that the genes must be small particles located in the chromosome pair, and the other member of the gene pair in the other?

Answer 10

He realised that the number of gene pairs was obviously much greater than the number of pairs of chromosomes.

Question 11

What are homologous chromosomes ?

Answer 11

Chromosome pairs which contains genes that control the same characteristics.

Question 12

What is a gene linkage ?

Answer 12

It is a situation in which genes in a particular chromosome are all linked together to form a set.

Question 13

Define Locus?

Answer 13

The point in a chromosome occupied by a gene

Question 14

Define Alleles?

Answer 14

• An alternative form of a gene.
• Each gene may have two or more alleles.
• Since each person has two genes for a characteristic, they have two alleles, which may be the same or different.

Question 15

What are multiple alleles?

Answer 15

• When there is more than two alleles for a particular characteristic.
• The locus of that gene is referred to as multi-allelic.

Question 16

Define Dominance.

Answer 16

An allele that masks the effect of another allele.

Question 17

Define Recessive.

Answer 17

An allele that is masked by the effect of an alternative allele.

Question 18

Define Codominance.

Answer 18

Of a pair of alleles, neither masks the effect of the other and both are expressed in the phenotype of a heterozygous individual.

Question 19

Define Homozygous.

Answer 19

The situation where an individual has the same alleles for a particular characteristic; also called pure-breeding.

Question 20

Define Heterozygous.

Answer 20

The situation where an individual possesses different alleles for a particular characteristic; also called hybrid.

Question 21

Define Phenotype.

Answer 21

The observable structure, function or behaviour of an individual as determined by the expression of the alleles for that characteristic and the interaction of the environment.

Question 22

Define Genotype.

Answer 22

The genetic make-up of an individual.

Question 23

Define monogenic inheritance.

Answer 23

When the pattern of inheritance is still one of discrete characteristics and because each person has only one pair of alleles for the characteristic.

Question 24

Define polygenic inheritance.

Answer 24

• A far more common situation, where individuals show continuous variation; a person may have any characteristic from one extreme to another.
• In these cases, the inheritance of the characteristic depends on many pairs of alleles, not just one pair.
• Examples are skin colour, height or body build in humans.

Question 25

What are factors that may influence polygenic inheritance?

Answer 25

-Environmental factors as diet, exercise, disease-causing organisms and the concentration of ultraviolet light can influence the phenotype of an individual.

Question 26

What are the polygenes that represent white and black skin?

Answer 26

Black Skin - AABBCC | White Skin - aabbcc

Question 27

What does the darkness of the phenotype depend on ?

Answer 27

It depends on the number of capital letters in the genotype.

Question 28

What is another human characteristic displaying continuous variation?

Answer 28

- Height | - Eye colour

Question 29

What are factors that may affect height.

Answer 29

- Diet would be a major environmental influence on the expression of the characteristic. - This is well represented by the differences in height between North and South Koreans.

Question 30

What does eye colour depend on?

Answer 30

It depends on the presence and distribution of pigment.

Question 31

Define Melanin.

Answer 31

A yellow-black pigment produced in special skin cells called melanocytes.

Question 32

What does the skin colour of an individual mainly depend on?

Answer 32

The amount of melanin produced.

Question 33

What are Melanocytes?

Answer 33

Special skin cells which produce structures called melanosomes that contain melanin.

Question 34

Where does Melanin form?

Answer 34

Melanin forms in melanosomes from an amino acid called tyrosine, the synthesis of which is catalysed by a number of enzymes.

Question 35

Explain the structure of the Melanocytes.

Answer 35

- Melanocytes are present in similar numbers in the skin of all humans, whether they are fair or dark. - However, within the melanocytes, the melanosomes vary. - In individuals with dark skin, they are larger, contain more melanin and are more evenly distributed.

Question 36

What affects the variation in melanin ?

Answer 36

Much of the variation in melanin appears to be due to variation in two major genes that affect the enzyme tyrosinase, which in turn affects melanin production.

Question 37

Individuals who lack the enzymes of a particular process will produce little or no melanin, and thus have fairer skin. What process is this?

Answer 37

It is one of the biochemical processes occurring in melanocytes will produce large amounts of melanin when stimulated by the ultra-violet light of the sun.

Question 38

How does the environment affect the expression of the genes for skin colour?

Answer 38

All but the darkest skins will produce more melanin when exposed to ultraviolet light.

Question 39

What is another environmental factor which we are all exposed to?

Answer 39

-Diet; affects weight and height.

Question 40

How does Diet affect height?

Answer 40

- The genetic potential for height that people inherit from their parents is affected by diet. - A diet deficient in essential proteins and minerals may result in a person not achieving their potential height.

Question 41

How does Diet affect weight?

Answer 41

- Excessive food intake may have negative effects that result in a person being much larger than expected when looking at their family background. - This condition of being overweight or obese usually results when a person eats food that contains more energy than the body is able to use. - Such foods are usually high in carbs and fats, and the person stores the additional energy in fat storage cells known as adipose tissue.

Question 42

How can exercise affect gene expression?

Answer 42

- People involved in the highest levels of sport train regularly to build up particular muscle groups. - Swimmers at national and Olympic level frequently have very broad shoulders and an enlarged lung capacity developed from their specialised training programs. - Elite athletes, such as shot putters, train to build up the muscle groups necessary to excel in their chosen event.

Question 43

Explain how the environment can affect the expression of genes through Intellectual Development.

Answer 43

1. Diet; a generally poor diet can retard mental development but the absence of specific substances in the diet can have the same effect. 2. A lack of stimulation during infancy can also lead to poor development of intellectual skills. - Babies who are not stimulated by interaction with adults, or do not have toys and other objects to explore, will not develop to their full potential.

Question 44

What is DNA?

Answer 44

- Deoxyribonucleic acid is found in the cells of all organisms, usually in the nucleus. - The molecules of DNA contain the genetic information that determines the structure of the cell and the way it functions.

Question 45

What has to happen for the growth of an organism to occur?

Answer 45

- Before the cell divides the DNA must be copied. - DNA molecules undergo replication, a process in which they form exact copies of themselves. - They are in the form of long strands which, when the cells is about to divide, become tightly coiled structures called chromosomes. - The genes contain the genetic code, which is the stored information that determines the structure and the activities of the cell.

Question 46

What is gene expression?

Answer 46

It is the process of copying information from DNA onto messenger RNA (mRNA) and then translating the message into a series of amino acids to form a protein.

Question 47

Give a simple summary of gene expression.

Answer 47

1. DNA - > The store of information - the genes 2. Transcription. 3. Messenger RNA - > Carrier of the tinformation 4. Translation 5. Protein - > The product of expression of the information.

Question 48

Define Variaitons.

Answer 48

Variations are the differences that occur between members of the same species. -All humans belong to the species Homo Sapiens and we all share certain basic characteristic, but at the same time we are all different from one another.

Question 49

What are the two sources of variation?

Answer 49

1. Inheritance 2. Environment 3. Gene expression

Question 50

How is gene expression a form of variation?

Answer 50

- Differences in the level of gene expression can contribute to phenotypic variation among individuals. - Depending on whether a gene is expressed or suppressed, the phenotype may vary without any change in the DNA.

Question 51

Define Epigenetic.

Answer 51

-Epi means 'on top of', or 'in addition to', so epigenetics refers to changes in gene expression that result from mechanisms other than changes in DNA.

Question 52

What is one way that are genes are regulated epigenetically?

Answer 52

Through Chromatin.

Question 53

What is a Chromatin?

Answer 53

- The DNA molecules are coiled around special proteins called histones. - When a cell is not dividing, the coiled DNA forms a tangled network. - The DNA and the histone proteins associated with it are called chromatin.

Question 54

What are some ways that Gene Expression may change?

Answer 54

1. If the way in which the DNA is wrapped around the histone changes 2. Acetylation 3. Chromatin remodelling

Question 55

Give an example of gene expression changing when the way in which the DNA is wrapped around the histone changes.

Answer 55

For eg. Some of the amin acids that are in the histone proteins may be changed. - This could change the shape of the histone and the modified histone shape may be copied each time a new DNA molecule is formed. - The modified histone would ensure that a stem cell that differentiated into a liver cell would remain a liver cell and not revert to being a stem cell.

Question 56

What is Acetylation?

Answer 56

- It is another histone modification. - The addition of an acetyl group (CH3CO) to the histone protein. - Acetylation enhances gene expression.

Question 57

What is Chromatin Remodelling?

Answer 57

- Occurs when adding methyl groups to the DNA molecule. ( A methyl group has one carbon atom attached to three hydrogen atoms: CH3) - This DNA methylation usually occurs at sites on the DNA molecule where a cytosine nucleotide is adjacent to a guanine nucleotide. - These are known as CpG sites, cytosine-phosphorous-guanine. - Methylation of DNA inhibits gene expression.

Question 58

What is the difference between Monozygotic and Dizygotic twins?

Answer 58

Monozygotic Twins: They develop from a single zygote - a single egg fertilised by a single sperm. For some reason the zygote divides and develops into two individuals who have identical DNA. Dizygotic Twins: When two eggs may be fertilised by two sperm at the same time, resulting in two twins who have only about 50% of their DNA in common.

Question 59

How do scientists unravel the influence on the development of a person of genetics versus environment?

Answer 59

- Scientists have compared monozygotic and dizygotic twins. - > Monozygotic twins have the same DNA and they also usually share the same environment before birth and childhood. - Dizygotic twins serve as the control because they share the same early environmental factors but, genetically, they are no more alike than ordinary brothers and sisters because they do not have the same DNA.

Question 60

Why do some genes become active in one twin and not the other, and why are some genes suppressed in one twin and not the other?

Answer 60

In an effort to isolate the epigenetic factors involved, twin studies have begun to focus on identical twins who were separated at an early age, and thus exposed to quite different environments.