Inheritance

Question 1

What is the gene locus?

Answer 1

Position occupied by the gene on the chromosome
(Plural : loci)

Question 2

What is an allele?

Answer 2

Alternative forms of a gene, each having a slightly different nucleotide sequence which gives rise to slightly different polypeptides

Question 3

What is a dominant allele ?

Answer 3

An allele whose characteristic is expressed in the phenotype even in the presence of an alternative allele

Question 4

What is a recessive allele ?

Answer 4

Allele whose characteristics is only expressed in the phenotype in the presence of another identical allel

Question 5

What is genotype - heterozygous and homozygous ?

Answer 5

Genotype : genetic makeup of an organism or a set of alleles of an organism
Homozygous : diploid condition where both allele are identical
Heterozygous : diploid condition where different alleles are present

Question 6

what is phenotype?

Answer 6

All the characteristics of an individual that are expressed

Question 7

What does pure bred / true bred mean?

Answer 7

Organism is homozygous for a particular characteristic or trait and will hence give rise to offspring having the same characteristics as itself
- both recessive and dominant are counted

Question 8

What is the definition of co-dominance ?

Answer 8

Both alleles of the pair in a heterozygote are expressed in the phenotype
(Both characteristics are expressed - spotted)

Represent in genetic diagram :
gene - uppercase
allele - superscript uppercase

Question 9

What is the definition incomplete dominance ?

Answer 9

Neither allele is completely dominant and offspring has phenotype on between that of the two varieties
(Intermediate trait between homozygous recessive and homozygous dominant is expressed : red+white=pink)

Represent in genetic diagram (same as co-dominant) :
gene - uppercase
allele - superscript uppercase

Question 10

What is a test cross?

Answer 10

Cross between an organism with an unknown genotype (showing the dominant characteristics) with a homozygous recessive individual

Question 11

What is a back cross?

Answer 11

Cross between unknown genotype (w dominant characteristics) with parent individual with homozygous recessive

Question 12

(Monohybrid) what is the expected phenotypic ratio between 2 heterozygotes?

Answer 12

3:1

Question 13

(Dihyrbid) what is the expected phenotypic ratio between two heterozygotes?

Answer 13

A_B_ : A_bb : aaB_ : aabb
9 : 3 : 3 : 1

Question 14

What is the definition of multiple alleles?

Answer 14

A trait is controlled by a gene which has 3 or more alleles of which only 2 may occupy the same locus on a pair of homologous chromosomes
- eg ABO blood group

Question 15

What are sex chromosomes?

Answer 15

Chromosomes that determine the sex of the organism

Question 16

What are autosomes?

Answer 16

The rest of the chromosomes that do not determine sex

Question 17

What are sex-linked genes?

Answer 17

Genes that are found on the sex chromosome and hence their expression will be affected by the sex of the organism

Question 18

What are sex-linked traits (sex linkage) ?

Answer 18

Characteristics in which the genes involved are found on the sex chromosomes the inheritance of this trait is then dependent on the sex of the organism
- eg. Haemophilia, red-green colour blindness

Question 19

What is a reciprocal cross?

Answer 19

Cross where 2 crosses are done where the same genetic genetic features are used but the sexes are reversed for each cross
Purpose : evaluate importance of parental sex on the inheritance of given characteristic

If results of reciprocal cross are different - sex linked genes

Question 20

[summary] sex-linkage
Clues for sex linked
1. Reciprocal cross gives different results in F1
2. Phenotypic ratios differ in male and female progeny of a cross
3. A trait shows “crisscross” inheritance - mother to son inheritance
4. Trait more common in heterogametic sex than in homogametic sex - males are more affected than females

Question 21

What is Haemophilia - bleeder’s disease ?

Answer 21

Inability to produce one of the many clotting factors (factor 8) which increases rate of blood clotting

Question 22

(Pedigree) What to look out for in autosomal dominant inheritance ?

Answer 22

Clues
1. Trait occurs with equal frequency in males and females
2. When both parents do not show the trait, none of their children will show it either
3. When a heterozygote marries an unaffected spouse, half the children will show the trait
4. Trait never skips a generation

Question 23

(Pedigrees) what to look out for in autosomal recessive inheritance?

Answer 23

Clues
1. Occurs with equal frequency in both genders
2. Affected children may have phenotypically normal parents
3. When both parents are carriers, 1/4 of their children show the trait
4. Skips generations in a direct ancestral line

Question 24

(Pedigrees) what to look out for in sex linked recessive inheritance?

Answer 24

Clues
1. More males develop their trait than females
2. Absence of father-to-son transmission
3. Half of the sons of carrier mothers develop the trait
4. Trait skips generations

Question 25

(Pedigrees) what to look out for in sex linked dominant inheritance ?

Answer 25

Clues 1. Both genders are affected 2. Easily confused w autosomal dominant inheritance but one important different : affected females (carriers) transmit the trait to none of his sons and to all his daughters 3. Trait does not skip generations

Question 26

What is autosomal linkage?

Answer 26

Occurs when two separate genes are found on the same chromosome Alleles of the two genes do not assort independently but are inherited together if crossing over does not occur - genetic diagram : linked genes are represented together with a line for chromosome

Question 27

What is the autosomal linkage phenotypic ratio?

Answer 27

Does not follow expected Mendelian ratio of 9:3:3:1 Higher proportion of individuals showing the parental phenotype (of crossing over occurs)

Question 28

Crossing over = recombinants How to determine distance between two linked genes (using proportion of recombinants) ?

Answer 28

Cross over value (COV)/Recombination frequency = no. of recombinants / total number of offspring x 100% Distance between two linked genes that give 1% COV = 1 map unit = 1 centimorgan (cM)

Question 29

What is epistasis?

Answer 29

One gene (epistatic) masks or modifies the phenotype expression of another gene (hypostatic)

Question 30

What is recessive epistasis?

Answer 30

Recessive genotype of one gene (aa) suppresses the expression of another (B/b)

Question 31

What is recessive epistasis phenotypic ratio?

Answer 31

9:3:4 aaB_ and aabb have same phenotype (3+1)

Question 32

What is dominant epistasis?

Answer 32

The dominant genotype at one gene (A_) produces a certain genotype regardless of the allelic condition of the other (B/b)

Question 33

What is dominant epistasis phenotypic ratio?

Answer 33

12 : 3 : 1 A_B_ and A_bb have the same phenotype (9+3)

Question 34

What is Complementary gene action?

Answer 34

(Enzymatic pathway) Epistatic interaction where both genes are required for the expression of the phenotype - >2 phenotypes

Question 35

What is duplicate genes?

Answer 35

(Enzymatic pathway) Genes at two diff loci make enzymes that catalyse the same reaction - presence of any one active enzyme will result in different phenotype - 2 phenotypes

Question 36

What is Complimentary gene action phenotypic ratio?

Answer 36

9 : 7 A_bb, aaB_, aabb gives same phenotype (3+3+1)

Question 37

What is Duplicate genes phenotypic ratio?

Answer 37

15 : 1 A_B_, A_bb, aaB_ gives same phenotype (9+3+3)

Question 38

What is a chi-square test (X power 2) ?

Answer 38

A statistical test that compares differences/deviations between the observes and expected data - determines whether the observed data conforms to the expected (or theoretical) data

Question 39

What is the null hypothesis?

Answer 39

Negative hypothesis stating that there are no significant differences between the observed and expected data Any difference is purely due to chance Observed data follows expected Mendelian ratio of …

Question 40

What does it mean to reject or not reject the null hypothesis?

Answer 40

Reject : significant differences between observes and expected data Not reject : no significant differences between the observed and expected data

Question 41

How to read the table of probability ?

Answer 41

Top row : probability level - probability that differences are due to chance - fixed at 0.05 - > 0.05 = due to chance - <0.05 = significant factors, not chance Left-most column : degree of freedom - number of phenotypic classes minus 1

Question 42

How to compare theoretical chi-square value with calculated value?

Answer 42

Find theoretical value by matching the degree of freedom and 0.05 fixed probability If calculated > theoretical : significant differences between observed and expected If calculated < theoretical : no significant difference between observed and expected

Question 43

How to compare probability level?

Answer 43

Obtain probability from calculated value by slotting the value into the table If probability > 0.05 : no significant differences between observed and expected If probability < 0.05 : significant differences between observed and expected

Question 44

What is variation?

Answer 44

Differences in characteristics observed in organisms of the same natural population or species

Question 45

What are the four sources of variation?

Answer 45

1. Environment - affect gene expression, give rise to variation in the phenotype 2. Gene reshuffling - reshuffling of existing alleles due to meiosis and random fertilisation 3. Gene mutation - change in DNA sequence resulting in formation of new alleles coding for new variation of the trait 4. Chromosomal aberration - changes in structure of a chromosome or number of chromosome

Question 46

What are the two types of variation?

Answer 46

1. Discontinuous variation 2. Continuous variation

Question 47

What is discontinuous variation ?

Answer 47

Variation in which individuals show clear-cut differences (fall into distinct classes) with no intermediates - the frequency histogram of a the particular characteristic has separate bars Characteristic is controlled by 1 or 2 genes with 2 or more allelic forms Phenotypic expression relatively unaffected by environment (there are exceptions)

Question 48

What is continuous variation ?

Answer 48

Variation in which individuals show a complete gradation from one extreme to another without a break - frequency histogram of the characteristic shows a smooth curve Characteristic is controlled by the additive effect of many genes (polygenes) with each gene contributing a certain degree to the particular phenotype Usually affected by the environment (not a must)