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Define the term “variation”.

The differences in characteristics between organisms.


Define the term “interspecific variation”.

The differences between organisms of different species.


Define the terms “intraspecific variation”.

The differences between organisms of the same species.


Name and describe the two causes of variation.

1) Environmental -
The environment in which the organisms lives causes environmental differences.

2) Genetic -
The differences in the genetic material an organism inherits from its parents leads to genetic variation.


Describe 5 causes of genetic variation between individuals within a population.

- Alleles:
with a gene for a particular characteristic, different alleles produce different effects and different individuals within a species population may inherit different alleles of a gene.
- Mutation:
Changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for.
- Meiosis:
Gametes are produced by meiosis. Each gamete receives half the genetic content of a parent cell and the genetic material is mixed up by independent assortment and crossing over.
- Sexual reproduction:
Offspring produced from two individuals inherits genes from each parent.
- Chance:
Different gametes are produced and in reproduction, it is chance as to which two combine. This is why individuals differ from siblings.


Define the term “phenotype".

Observable characteristics of an organism.


Define the term "genotype".

The genetic makeup of an organism.


Describe how chlorosis in plants is an example of phenotypes influenced by both genetic and environmental factors.

Chlorosis results in the leaves appearing pale or yellow because their cells are not producing the normal amount of chlorophyll. Most plants which show chlorosis have the normal genes coding for chlorophyll production.

Change in phenotype is due to environmental factors:

- Lack of light, the absence of light causes plants to turn off chlorophyll production to save energy.
- Mineral deficiencies, a lack of iron or magnesium. Iron is a cofactor needed by enzymes to produce chlorophyll.
- Virus infections; when viruses infect plants, they interfere with metabolism of cells.


Describe how body mass in animals is an example of
a phenotype influenced by both genetic and environmental factors.

- Dramatic variations in weight are generally due to diet, presence of a disease and level of exercise (environmental).
- Obesity can be the results of a mutation in your genes which causes the pattern of fat depositions to be altered


Define the term “allele”.

Different version of the same gene.


Define the term “dominant allele”.

The version of the gene that will always be expressed if present.


Define the term “recessive allele”.

The version of the gene that will only be expressed if two copies are present in the organism.


Define the term “homozygous”.

Two identical alleles for a characteristic.


Define the term “heterozygous”.

Two different alleles for a characteristic.


Define the term “carrier”.

A person who has one copy of a recessive allele coding for a genetically inherited condition.


Define the term “monogenic inheritance”.

A characteristic inherited on a single gene.


Define the term “dihybrid inheritance”.

A characteristic inherited on two genes.


Define the term “autosomal linkage”.

Genes present on the same non-sex chromosome.


Define the term “sex linked genes”.

Genes carried on the sex chromosomes.


Define the term “codominance”.

When different alleles of a gene are equally dominant and both are expressed in the phenotype.


Define the term “epistasis".

The effect of one gene on the expression of another gene.


Describe the 6 steps for drawing a genetic cross diagram

1) State the phenotype of both the parents.
2) State the genotype of both parents. Assign a letter to represent each of the alleles of the genes being studied. Capitalised for dominant and lower case for recessive.
3) State the gametes of each parent and circle them.
4) Use a Punnett Square to show the results of the random fusion of gametes on the edges of the square.
5) State the proportion of each genotype which are produce among the offspring. Can be in the form of a percentage or ratio.
6) State the corresponding phenotype for each of the possible genotypes.


Describe what co-dominance is.

Co-dominance occur when two different alleles occur for a gene which are both equally dominant. As a result, both alleles of the gene are expressed in the phenotype of the organism if they are present.
e.g. colour of flowers, red and white made pink.


How is co-dominance represented in genetic diagrams?

Upper and lower case letters are not used to represent the genes as this would imply dominant and recessive.
Instead, a letter is chosen to represent the gene, and the different alleles are then represented using a second letter and is shown as a superscript.


Describe what happens when there are multiple alleles for a gene.

- Some genes have more than two versions of alleles, they have multiple alleles .
- However, an organism only carries two versions of a particular gene (one on each of the homologous chromosomes) so only two alleles can be present in an individual.
- Blood group is determined by a gene with multiple alleles. This results in many different possible crosses.


Describe how sex is determined in humans.

- Genetically determined.
- Humans have 23 pairs of chromosomes, and in 22 of the pairs both chromosomes are the same.
- The 23rd pair, know as the sex chromosomes, are different.
- Human females have two X chromosomes (XX) whereas a male has an X and a Y (XY).
- The X chromosome is large and contains many genes not involved in sexual development. The Y chromosome is very small, and carries almost no genetic info except the genes that causes the embryo to develop into a male.
- Therefore, the sex of an offspring will be determined by whether the sperm fertilising the egg contains an X or a Y chromosome.


Describe what sex linkage.

- Sex linked characteristics are characteristics determined by genes carried on the sex chromosomes.
- As the Y is much smaller than X chromosome, there are many genes in the X chromosome that males only have one copy of (because they are no present in the Y chromosome).
- This means that any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males.
- This because many females will also have a dominant allele present on their second X chromosome, which overrides the recessive allele.


Give an example of a sex-linked genetic disorder.

Haemophilia - the absence of clotting factor that results in pro-longed bleeding following an injury.

It is caused by a recessive allele on the X-chromosome, so most haemophilia suffers are male. A female would have to be heterozygous recessive for it to be expressed.


Describe what dihybrid inheritance and how a dihybrid cross would look.

- A dihybrid cross is used to show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.
- In a dihybrid cross four alleles (two for each characteristic) are shown at each stage instead of two.


State the expected phenotypic ratio for dihybrid crosses involving: i) two double heterozygotes, ii) one double heterozygote and one double homozygous recessive.

--- ?? See sophie's, ask guilly.