Inheritance Flashcards
What is the gene locus?
Position occupied by the gene on the chromosome
(Plural : loci)
What is an allele?
Alternative forms of a gene, each having a slightly different nucleotide sequence which gives rise to slightly different polypeptides
What is a dominant allele ?
An allele whose characteristic is expressed in the phenotype even in the presence of an alternative allele
What is a recessive allele ?
Allele whose characteristics is only expressed in the phenotype in the presence of another identical allel
What is genotype - heterozygous and homozygous ?
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
what is phenotype?
All the characteristics of an individual that are expressed
What does pure bred / true bred mean?
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
What is the definition of co-dominance ?
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
What is the definition incomplete dominance ?
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
What is a test cross?
Cross between an organism with an unknown genotype (showing the dominant characteristics) with a homozygous recessive individual
What is a back cross?
Cross between unknown genotype (w dominant characteristics) with parent individual with homozygous recessive
(Monohybrid) what is the expected phenotypic ratio between 2 heterozygotes?
3:1
(Dihyrbid) what is the expected phenotypic ratio between two heterozygotes?
A_B_ : A_bb : aaB_ : aabb
9 : 3 : 3 : 1
What is the definition of multiple alleles?
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
What are sex chromosomes?
Chromosomes that determine the sex of the organism
What are autosomes?
The rest of the chromosomes that do not determine sex
What are sex-linked genes?
Genes that are found on the sex chromosome and hence their expression will be affected by the sex of the organism
What are sex-linked traits (sex linkage) ?
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
What is a reciprocal cross?
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
[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
What is Haemophilia - bleeder’s disease ?
Inability to produce one of the many clotting factors (factor 8) which increases rate of blood clotting
(Pedigree) What to look out for in autosomal dominant inheritance ?
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
(Pedigrees) what to look out for in autosomal recessive inheritance?
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
(Pedigrees) what to look out for in sex linked recessive inheritance?
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
(Pedigrees) what to look out for in sex linked dominant inheritance ?
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
What is autosomal linkage?
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
What is the autosomal linkage phenotypic ratio?
Does not follow expected Mendelian ratio of 9:3:3:1
Higher proportion of individuals showing the parental phenotype (of crossing over occurs)
Crossing over = recombinants
How to determine distance between two linked genes (using proportion of recombinants) ?
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)
What is epistasis?
One gene (epistatic) masks or modifies the phenotype expression of another gene (hypostatic)
What is recessive epistasis?
Recessive genotype of one gene (aa) suppresses the expression of another (B/b)
What is recessive epistasis phenotypic ratio?
9:3:4
aaB_ and aabb have same phenotype (3+1)
What is dominant epistasis?
The dominant genotype at one gene (A_) produces a certain genotype regardless of the allelic condition of the other (B/b)
What is dominant epistasis phenotypic ratio?
12 : 3 : 1
A_B_ and A_bb have the same phenotype (9+3)
What is Complementary gene action?
(Enzymatic pathway)
Epistatic interaction where both genes are required for the expression of the phenotype
- >2 phenotypes
What is duplicate genes?
(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
What is Complimentary gene action phenotypic ratio?
9 : 7
A_bb, aaB_, aabb gives same phenotype (3+3+1)
What is Duplicate genes phenotypic ratio?
15 : 1
A_B_, A_bb, aaB_ gives same phenotype (9+3+3)
What is a chi-square test (X power 2) ?
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
What is the null hypothesis?
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 …
What does it mean to reject or not reject the null hypothesis?
Reject : significant differences between observes and expected data
Not reject : no significant differences between the observed and expected data
How to read the table of probability ?
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
How to compare theoretical chi-square value with calculated value?
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
How to compare probability level?
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
What is variation?
Differences in characteristics observed in organisms of the same natural population or species
What are the four sources of variation?
- Environment - affect gene expression, give rise to variation in the phenotype
- Gene reshuffling - reshuffling of existing alleles due to meiosis and random fertilisation
- Gene mutation - change in DNA sequence resulting in formation of new alleles coding for new variation of the trait
- Chromosomal aberration - changes in structure of a chromosome or number of chromosome
What are the two types of variation?
- Discontinuous variation
- Continuous variation
What is discontinuous variation ?
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)
What is continuous variation ?
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)
