genetics and inheritance

Question 1

What is a gene?

Answer 1

A gene is a length of DNA that codes for a particular polypeptide or protein. It is a unit of inheritance.

Question 2

What is an allele?

Answer 2

An allele is one of 2 or more alternative nucleotide sequences at a single gene locus, they are variant forms of genes

Question 3

What is a locus?

Answer 3

Locus is the position on the chromosomes where a gene occurs, alleles of the same gene occupy the same locus

Question 4

What is a genotype?

Answer 4

It is the alleles that an organisms carries

Question 5

What is a phenotype?

Answer 5

Phenotype is the appearance of the organism and the way in which the genotype of the organism is expressed

Question 6

What is homozygous and heterozygous?

Answer 6

Homozygous: a genotype in which the 2 alleles of a gene are the same (e.g. HH or hh)
Heterozygous: a genotype in which the 2 alleles of a gene are different (e.g. Hh)

Question 7

What is a dominant and recessive allele?

Answer 7

Dominant allele: an allele with a phenotype that is expressed even when present with an allele that is recessive
Recessive allele: an allele with a phenotype that is not expressed when an allele that is dominant to it is present

Question 8

What is monohybrid cross?

Answer 8

It is the investigation of the inheritance of a single contrasting characteristic (e.g. tall and dwarf plants, where F1 gen is all tall and F2 gen has tall: dwarf in the ratio of 3:1)

Question 9

What is the law of segregation?

Answer 9

There are 2 alleles (alternative form) of the gene in each cell, one from each parent. Each has an equal chance of being passed on to an offspring but one is dominant over the other allele.

Question 10

What is a test cross?

Answer 10

Mating between an individual of dominant phenotype (but unknown genotype) and a homozygous recessive individual

Question 11

What are the 5 steps in genetic diagram questions?

Answer 11

1. key (let N represent the allele for, let n represent the allele for…) (NO dominant and recessive)
2. parental generation (PGGametes)
3. F1 generation (table, gametes random fertilisation, (circle gametes), P (G) P (G)
4. F1 offspring (P P ratio G G ratio)
5. concluding statement (probability expressed as a fraction)

Question 12

What are some modifications to the 3:1 ratio?

Answer 12

Besides complete dominance (P p),
Codominance (heterozygous: both phenotypes expressed - sickle cell anaemia)
Incomplete dominance (heterozygous: intermediate between the two homozygous phenotypes - C^R C^W)
Multiple Alleles (three or more alleles that can occupy the same locus - blood type - i^A i^B i)
Sex linkage - (X^A Y X^a Y vs X^A X^A X^a X^A X^a X^a)

Question 13

How many alleles does dihybrid cross contain?

Answer 13

4 alleles (it is the investigation of the inheritance of 2 pairs of contrasting characters)

Question 14

Ratio for dihybrid cross?

Answer 14

R_Y_ : R_yy : rrY_ : rryy
9: 3: 3: 1

Question 15

What is the law of independent assortment?

Answer 15

During meiosis the separation of one pair of alleles is independent of the separation of another pair of alleles.

Question 16

What are the two types of variation? Explain briefly about each one

Answer 16

Discontinuous variation: Characteristic concerned is one of two or more discrete types with no intermediate forms
Continuous variation: Continuous distribution of values

Question 17

What are some examples of discontinuous and continuous variation?

Answer 17

Discontinuous variation: blood type
Continuous variation: Melanin synthesis (skin colour)

Question 18

What is the genetic basis of discontinuous variation?

Answer 18

It is controlled by 1 gene with 2 alleles. There is a clear cut difference in an inherited characteristic with no intermediate form and no overlap between both phenotypes.

Question 19

What is the genetic basis of continuous variation?

Answer 19

It is controlled by a number of genes (polygenes). Effects on any one of these genes make a very small or insignificant effect on the phenotype, but the combined effect of all the genes is to produce infinite variety among the offspring.