chapter 20 p1

Question 1

Variation

Answer 1

Members of different species are, usually, clearly different from each other and even members of the same species are rarely identical so variation is an important feature of living organisms.
Variation arises as a result of mutations - changes to the genetic code which are random and constantly taking place.
Variation is essential for the process of natural selection - and therefore evolution.
variation can occur both as a result of environmental variation and genetic variation.
In the majority of cases both play a role in determining an organism’s characteristics - examples of this include chlorosis in plants and the body mass of an animal.

Question 2

Chlorosis:

Answer 2

• Most plants are genetically coded to produce large quantities of chlorophyll, the green pigment that is vital for photosynthesis and gives leaves their green colour.
• Some plants, however, suffer from a condition known as chlorosis, when the leaves look pale or yellow.
• This occurs because the cells are not producing the normal amount of chlorophyll.
• This lack of chlorophyll reduces the ability of the plant to make food by photosynthesis.
• Most plants which show chlorosis have normal genes coding for chlorophyll production.
• The change in their phenotype is the result of environmental factors.
• There are many different environmental factors which cause chlorosis, each having a different effect on the physiology of the plant but causing the same change in phenotype.

Question 3

Examples of environmental factors which cause chlorosis include:

Answer 3

Lack of light
Mineral deficiencies
Virus infections

Question 4

Lack of light

Answer 4

for example, when a toy or gardening tool is left on a lawn. In the absence of light, plants will turn off their chlorophyll production to conserve resources.
In this case, chlorosis only occurs where the plant gets no light.

Question 5

Mineral deficiencies

Answer 5

for example, a lack of iron or magnesium.
Iron is needed as a cofactor by some of the enzymes that make chlorophyll, and magnesium is found at the heart of the chlorophyll molecule.
If either of these elements are lacking in the soil, a plant simply cannot make chlorophyll and gradually all the leaves will become yellow.

Question 6

Virus infections

Answer 6

when viruses infect plants, they interfere with the metabolism of cells.
A common symptom is yellowing in the infected tissues as they can no longer support the synthesis of chlorophyll.

In summary, even though genetic factors in a plant are likely to code for green leaves, the environment plays a key role in the final leaf appearance.

Question 7

Animal body mass:

Answer 7

• Within a species, the body mass of individual animals varies.
• An organism’s body mass is determined by a combination of both genetic and environmental factors.
• In the majority of cases dramatic variations in size such as obesity and being severely underweight are a result of environmental factors.
• For example, the amount (and quality) of foods eaten, the quantity of exercise which the organism gets, or the presence of disease can affect the body mass.
• Being extremely overweight or underweight can result in significant health problems for an animal.
• Occasionally obesity can be a result of the genetic make-up of an organism.
• The obese mouse in Figure 3 has a mutation on chromosome 7.
• This mutation causes the pattern of fat deposition in its body to be altered.
• Scientific studies have shown that this gene acts in conjunction with other genes that regulate the energy balance of the body, and as a result mice possessing the mutation grow 35-50% fatter by middle age than a normal mouse would.

Question 8

Creating genetic variation:

Answer 8

• Genetic variation is created by the versions of genes you inherit from your parents.
• For most genes there are a number of different possible alleles or variants.
• The individual mixture of alleles an organism inherits influences the characteristics they will display.
• This combination is determined by sexual reproduction involving meiosis (the formation of gametes), and the random fusion of gametes at fertilisation.
• This results in the vast genetic variation seen between individuals of the same species.
• For most genes in your body two alleles are inherited (one from each parent).
• These alleles may be the same or different versions of the gene.

Question 9

genetic make-up of an organism in respect of that gene

Answer 9

The combination of alleles an organism inherits for a characteristic is known as their genotype

Question 10

phenotype.

Answer 10

The observable characteristics of an organism

Question 11

modifications.

Answer 11

Any changes the environment makes to a person’s phenotype are not inherited

The actual characteristics that an organism displays are also often influenced by the environment.

Question 12

what in the gametes can be passed on to the offspring.

Answer 12

Only mutations (changes to the DNA) in the gametes can be passed on to the offspring.

Question 13

why It is not always possible to determine an organism’s genotype from its phenotype

Answer 13

• due to the dominance of particular alleles.
• A dominant allele is the version of the gene that will always be expressed if present in an organism.
• This means an individual showing the dominant characteristic in their phenotype could have one or two copies of the dominant gene - you can’t tell from their appearance.
• A recessive allele, however, will only be expressed if two copies of this allele are present in an organism.
• This means if an individual has a recessive phenotype, you also know their genotype - they must have two alleles coding for the recessive phenotype.

Question 14

Homozygous

Answer 14

• they have two identical alleles for a characteristic.
• The organism could be homozygous dominant (contain two alleles for the dominant phenotype) or homozygous recessive (contain two alleles for the recessive phenotype).

Question 15

Heterozygous

Answer 15

they have two different alleles for a characteristic.
In this case the allele for the dominant phenotype will be expressed.

Question 16

Continuous and discontinuous variation:

Answer 16

• The variation of a characteristic displayed within a species can be divided into two groups - those which show continuous variation, and those which show discontinuous variation.
• in discontinuous variation individuals fall into distinct groups (for example, blood groups) and normally only one gene is involved and the environment has little, if any, effect (Figure 7).
• In continuous variation there are two extremes, with every degree of variation possible in between (Figure 8).
• Examples would include your height or weight.
• Many genes will be involved, and the environment has a large effect.

Question 17

continuos variation

Answer 17

Question 18

discontinuous variation

Answer 18

Question 19

To explain how characteristics are inherited

Answer 19

you need to be able to show how genes are passed on from one generation to the next.
This is normally shown using a genetic cross.
Most commonly the inheritance of a single gene is shown, this is known as monogenic inheritance.
The basic laws by which characteristics are inherited were established by Gregor Mendel, a scientist and monk working in the 19th century.

Question 20

Performing a genetic cross:
There are a number of key steps you should follow when analysing a genetic cross.
This ensures that your diagram explains fully what is happening to the genes of an organism during fertilisation (and helps you to avoid making errors):
step 1-3

Answer 20

Step 1. - State the phenotype of both the parents.

Step 2 - State the genotype of both parents.
To do this, assign a letter code to represent the alleles of the gene being studied.
A capital letter should be used to represent the dominant allele and its lower case form to represent the recessive allele.
For example, if studying the inheritance of an animal’s fur colour, you may choose B to represent brown fur (dominant) and b to represent white fur (recessive).

Step 3 - State the gametes of each parent. It is common practice to circle the letters, for example. G.

Question 21

Performing a genetic cross:
There are a number of key steps you should follow when analysing a genetic cross.
This ensures that your diagram explains fully what is happening to the genes of an organism during fertilisation (and helps you to avoid making errors):
step 4-6

Answer 21

Step 4 - Use a Punnett Square to show the results of the random fusion of gametes during fertilisation.
Remember to label the gametes on the edges of the square.

Step 5 - State the proportion of each genotype which are produced among the offspring.
This can be in the form of a percentage. ratio, or ‘x out of y offspring …’.

Step 6 - State the corresponding phenotype for each of the possible
Genotypes.
It must be clear that you know which phenotype results from each genotype.

Question 22

Homozygous genetic cross:

Answer 22

• Mendel carried out many of his famous experiments on pea plants.
• Pea pods come in two colours - green and yellow.
• The cross in Figure 1 shows what happens when a homozygous green pea pod plant is crossed with a homozygous yellow pea pod plant.
• The allele for green pea pods is dominant
• Organisms that contain homozygous alleles for a particular gene are known as true breeding or pure breeding individuals.
• Therefore in this experiment Mendel was studying what happened when two true breeding individuals are crossed.
• All of the offspring are heterozygous.
• This means that all plants will have green pods as this is the dominant allele.
• These offspring are known as the F1 generation.

Question 23

diagram of Homozygous genetic cross:

Answer 23

Question 24

Heterozygous genetic cross:

Answer 24

The cross in Figure 2 shows what happens if you take two of the heterozygous offspring from the first generation and cross them together.
The offspring produced from this cross are known as the F2 generation.
Offspring will be produced in a ratio of three pea plants with green pods to one pea plant with yellow pods.

Question 25

heterozygous genetic cross

Answer 25

Question 26

study tip

Answer 26