Topic 8.1 Genetic Information

Question 1

Independent assortment

Answer 1

• Any number from none to all 23 chromosomes in your gametes could come from either your maternal or paternal chromosomes.
• Introduces a considerable genetic variation.

Question 2

Crossing over (recombination)

Answer 2

• A large multi-enzyme complexes ‘cut and join’ bits of the maternal and paternal chromatids together at the chiasmata.
• Leads to added genetic variation as many new combinations of alleles arise.

Question 3

Point or gene mutations

Answer 3

Caused by miscopying of just one or a small number of nucleotides (eg. substitution, deletion, insertion).

Question 4

Chromosomal mutations

Answer 4

Changes is positions of genes within the chromosomes (these are more likely to make a measurable change than a gene mutation).

Question 5

Whole chromosome mutations

Answer 5

When an entire chromosome is either lost during meiosis or duplicated in one cell by errors in the process (they usually have a major impact on the organism).

Question 6

Phenotype

Answer 6

The physical and chemical characteristics that make up the appearance of an organism.

Question 7

Genotype

Answer 7

The genetic information.

Question 8

Polygenic traits

Answer 8

• Most traits in living organisms are determined by several interacting genes.
• Eg. eye colour, weight and intelligence are determined by a number of different genes (and in many cases interactions with the environment add further variety).

Question 9

Myogenic (monohybrid) crosses

Answer 9

When one gene is considered at a time in a genetic cross, it is referred to as a monogenic cross.
(These are represented by Punnett squares).
–> The first generation of the cross is called the F1.
–>If individuals from F1 are crossed with each other, the next generation is called the F2.

Question 10

Representing inheritance

Answer 10

Two laws of inheritance:
1) Law of segregation- one allele is inherited from each parent.
2) Law of independent- different traits are inherited independently of each other.

Question 11

Test cross

Answer 11

Cross with homozygous recessive to see if a plant is heterozygous or homozygous dominant.

Question 12

The law of segregation: Mendel’s first law

Answer 12

One allele for each trait is inherited from each parent to give a total of two alleles for each trait.

Question 13

The law of independent assortment: Mendel’s second law

Answer 13

Different traits are inherited independently of each other.
(This means that the inheritance of dominant or recessive phenotype for one characteristic, has nothing to do with the inheritance alleles for other characteristics).

Question 14

Multiple alleles

Answer 14

• Some features have multiple alleles.
• This means there are more than two possible variants.

Question 15

Codominance

Answer 15

• Blood can be codominant.
• I^A and I^B are codominant.
• This means both alleles are expressed.
• Individual with I^A and I^B will have both antigens.

Question 16

Genetic chi squared test

Answer 16

• H0: There is no statistically significant difference between the numbers observed and the Mendelian predicted outcome.
• The observed values will be given.
• Use the expected ratio from the dihybrid cross/test cross etc for the expected value.
• To find the expected values, add together the observed values and divide that number by the ratios from the expected.
  —> In the chi squared table add the phenotype, observed and expected values, and the x^2 value once worked out.
• To find the degrees of freedom: (Number of categories - 1).
• Use this to find the critical value.
  –> If the result is higher than the critical value then we reject null hypothesis (the results do not follow the predicted ratio).
  –> If the result is lower than the critical value we accept null hypothesis (the results follow the predicted ratio).
• Conclusion: There is/ is no statistically significant difference between….

Question 17

Digenic (dihybrid) crosses

Answer 17

Two characteristics are determined by two difference genes on different chromosomes.
–> There are some occasions when the ratios are not what you expect:
-small sample size
-experimental error
-the process is random
-unexpected ratios

Question 18

Gene linkage (/autosomal linkage)

Answer 18

• When two or more genes that are being inherited are sited on the same chromosome.
• E.g when heterozygous the dominant/recessive are inherited together and inherited as if they were one unit.
• (not sex chromosomes).

Question 19

Identifying linked genes

Answer 19

• If genes are found on separate chromosomes they are linked and inherited.
• The tightness of the linkage is related to how close together the linked genes are on the chromosome.
• Genes that are very close together are less likely to be split during the crossing over stage of meiosis then genes that are further apart.

Question 20

Homogametic

Answer 20

Females contain two X chromosomes ∴ all her eggs contain an X chromosome.

Question 21

Heterogametic

Answer 21

Males have one X chromosome and one much smaller Y chromosome ∴ half of the sperm will contain X chromosomes and half will contain Y chromosomes.

Question 22

The albino trait

Answer 22

• Due to a mutant allele that prevents the formation of a normal enzyme in the cells.
• Albinism of this type is a recessive phenotype.
• The parents may appear normal ∴ they are both carriers of the albino allele.
• People with albinism often have poor vision and are at a higher risk of developing skin cancers, as they do not have the natural protective pigment melanin.

Question 23

Genetic pedigree diagrams

Answer 23

–> Squares: male
–> Circles: female
- Genetic pedigree diagram scan be useful in predicting which family members may be carriers of the genetic mutation, because they highlight carriers of a recessive phenotype.
- They are useful for identifying sex-linked traits.
- They can also be used for selective breeding in animals.

Question 24

Sex-linked diseases in humans

Answer 24

-A mother always donate an X chromosome to her son.
-The father always donates the Y chromosome.
-Because the Y chromosome is small and carries only genes that code for traits associated with maleness, any mutations in a gene on the X chromosome will affect the phenotype of offspring, even if the characteristic it codes for is recessive.

Question 25

Red-green colour blindness

Answer 25

-The ability to see colour is the result of multiple genes coding for different aspects; many of these are found on the X chromosome.
-Mutations in these genes can affect our ability to see in colour.
-It is more common in men than women as it is sex-linked.

Question 26

Haemophilia

Answer 26

-Haemophilia is a sex-linked trait.
-One if the proteins needed for the clotting of the blood is missing.
-The components of the blood clotting cascade are coded for by multiple genes; many of these genes are on the X chromosome (which is why its sex-linked).

Question 27

Haemophilia A

Answer 27

-Sex-linked condition.
-Involves the lack of clotting factor VIII.