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Inheritance Flashcards

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Biology 101 flashcards
1
Q

What is the gene locus?

A

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

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2
Q

What is an allele?

A

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

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3
Q

What is a dominant allele ?

A

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

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4
Q

What is a recessive allele ?

A

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

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5
Q

What is genotype - heterozygous and homozygous ?

A

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

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6
Q

what is phenotype?

A

All the characteristics of an individual that are expressed

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7
Q

What does pure bred / true bred mean?

A

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

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8
Q

What is the definition of co-dominance ?

A

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

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9
Q

What is the definition incomplete dominance ?

A

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

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10
Q

What is a test cross?

A

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

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11
Q

What is a back cross?

A

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

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12
Q

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

A

3:1

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13
Q

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

A

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

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14
Q

What is the definition of multiple alleles?

A

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

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15
Q

What are sex chromosomes?

A

Chromosomes that determine the sex of the organism

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16
Q

What are autosomes?

A

The rest of the chromosomes that do not determine sex

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17
Q

What are sex-linked genes?

A

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

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18
Q

What are sex-linked traits (sex linkage) ?

A

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

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19
Q

What is a reciprocal cross?

A

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

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20
Q

[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

A
21
Q

What is Haemophilia - bleeder’s disease ?

A

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

22
Q

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

A

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

23
Q

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

A

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

24
Q

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

A

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

25
Q

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

A

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

26
Q

What is autosomal linkage?

A

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

27
Q

What is the autosomal linkage phenotypic ratio?

A

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

28
Q

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

A

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)

29
Q

What is epistasis?

A

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

30
Q

What is recessive epistasis?

A

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

31
Q

What is recessive epistasis phenotypic ratio?

A

9:3:4

aaB_ and aabb have same phenotype (3+1)

32
Q

What is dominant epistasis?

A

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

33
Q

What is dominant epistasis phenotypic ratio?

A

12 : 3 : 1

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

34
Q

What is Complementary gene action?

A

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

35
Q

What is duplicate genes?

A

(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

36
Q

What is Complimentary gene action phenotypic ratio?

A

9 : 7

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

37
Q

What is Duplicate genes phenotypic ratio?

A

15 : 1

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

38
Q

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

A

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

39
Q

What is the null hypothesis?

A

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 …

40
Q

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

A

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

41
Q

How to read the table of probability ?

A

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

42
Q

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

A

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

43
Q

How to compare probability level?

A

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

44
Q

What is variation?

A

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

45
Q

What are the four sources of variation?

A
  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
46
Q

What are the two types of variation?

A
  1. Discontinuous variation
  2. Continuous variation
47
Q

What is discontinuous variation ?

A

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)

48
Q

What is continuous variation ?

A

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)

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