B7 Inheritance

Question 1

Heredity

Answer 1

Transmission of genetic characteristics (genes stored in chromosomes) from 1 generation to the next and the effects of this transmission

Question 2

Variation

Answer 2

Recognisable differences between individuals of the same species and between parents and offsprings

Question 3

Gene

Answer 3

Unit of inheritance located at a particular locus of a chromosome
Specific DNA nucleotide sequence which codes for RNA (mRNA, tRNA, or rRNA) or a polypeptide

Question 4

Locus

Answer 4

Specific location of a gene on a chromosome

Question 5

Allele

Answer 5

Alternative form of a gene at a particular gene locus
Responsible for determining contrasting traits of same character
All alleles of a gene determine the same character but each has a unique DNA nucleotide sequence, which may result in different phenotypes
Allele occur in pairs in a diploid cell although only one of the pairs is represented in a gamete

Question 6

Genotype

Answer 6

Complete genetic makeup/ allelic composition of an organism

Commonly used in reference to the paired alleles carried by an organism that give rise to a phenotype

Question 7

Phenotype

Answer 7

Physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment

Question 8

Wild-type

Answer 8

Most common allele / phenotype in nature (vs mutant allele / phenotype)

Question 9

Homozygous

Answer 9

Condition in which the alleles of a gene pair in diploid condition are identical
Organism with this condition is known as a homozygote, referred to as true or pure breeding
All gametes produced carry same allele

Question 10

Heterozygous

Answer 10

Alleles of a gene pair in diploid condition are different

Organism known as heterozygote

Question 11

Dominant alleles

Answer 11

Produce effects in both homo and heterozygous condition
Dominant allele masks the influence of recessive allele
Organism homozygous for dominant allele is known as homozygous dominant

Question 12

Recessive alleles

Answer 12

Produce effects only in homozygous condition

Homozygous recessive

Question 13

True-breeding

Answer 13

Organism is homozygous, all offspring of the same phenotype

Question 14

Carriers

Answer 14

Person or other organism that has inherited a recessive allele for a genetic trait or mutation but does not display trait / show symptoms of disease

Question 15

Link between genotype and phenotype

Answer 15

Alleles are transcribed to form mRNA and translated into different pp chains and proteins which may affect different metabolic pathways resulting in formation of different phenotypes. Each allele thus specifies or codes for the specific 3D conformation of a particular protein which directly determines a trait.
Genotype is genetic makeup of organism, which refers to paired alleles that produces phenotype which is a measurable or distinctive character

Question 16

Mendel’s First Law of Segregation

Answer 16

During formation of gametes, paired alleles segregate randomly so each gamete receives one of the other with equal likelihood.

Question 17

Incomplete Dominance

Answer 17

A condition where neither of the 2 alleles are completely dominant to the other, so heterozygote has phenotype which is intermediate
1:2:1

Question 18

Codominance

Answer 18

Both alleles equally expressed in phenotype of heterozygote

Heterozygote simultaneously expresses phenotypes of both types of homozygotes

Question 19

Multiple alleles

Answer 19

Gene controlling characteristic has 3 or more alleles

Question 20

Mendel’s Second Law of Segregation

Answer 20

Segregation of one pair of alleles is independent of segregation of other pairs

Question 21

Sex Linkage

Answer 21

Genes on sex (mostly X) chromosomes
Known as sex-linked genes / X-linked genes
Follow transmission pattern of X chromosome

Sex-linked inheritance mainly affects males as they are hemizygous for every sex-linked locus
Males possess only a single X chromosome, whatever allele present on X chromosome of males will be directly expressed in phenotype (even if recessive)

Question 22

Linked genes

Answer 22

Genes that control different characters and situated on the same chromosome at different loci
Inherited together
Complete / Incomplete linkage of linked genes
Chance of crossing over occurring between 2 linked genes is proportional to distance between them

Question 23

Complete Linkage

Answer 23

No crossing over between linked genes
All linked genes pass into same gamete, resulting in only parental gametes
Genes likely very close tgt on same chromosome, closely linked tgt
3:1 (for F1 of homo dom & homo rec, mating of F1 het offspring)

Question 24

Incomplete Linkage

Answer 24

Genes located some distance apart on same chromosome, can be separated when crossing over occurs
Genetic recombination when crossing over occurs, new combination of alleles in gametes
Offspring called recombinants
Offspring produced show a majority of parental allele combinations and hence parental phenotypes and a minority of recombinant allele combinations and hence recombinant phenotypes

Question 25

Heterozygotes - Linked genes arrangements

Answer 25

Coupling - 2 dom alleles on one chromosome, 2 rec alleles on one chromosome Repulsion - dom + rec on each chromosome

Question 26

Distance between genes based on proportion of recombinants

Answer 26

``` Crossover Value (COV) / Recombination Frequency (RF) : No of individuals showing recombination / Total no of offspring x100% By convention a COV of 1% represents a relative distance of 1 centimorgan (cM) on the chromosome ```

Question 27

Phenotypic Variance

Answer 27

VP = VG (genotypic variance) + VE (environmental variance) VE is most easily determined when homozygous organisms are studied. VG is due to genotype and includes effects of additive genes, dominant genes, and epistasis. Contributions of each to the phenotype can be estimated from crosses involving homozygous varieties and their F1, F2 and backcross progeny.

Question 28

Ways of introducing variation in genotypes - new combination of existing alleles, nOT new alleles!

Answer 28

(Asexual organisms - Variation almost always from environmental influences) 1. Gene Mutation (Inheritable change in nucleotide sequence of DNA, which occurs at a single gene locus on a chromosome, resulting in formation of new alleles) - Deletion, insertion or substitution of one or few nucleotides 2. Chromosomal Mutation Change in structure or number of chromosomes 3. Meiosis and Sexual Reproduction Crossing over of non-sister chromatids of homologous chromosomes during prophase I results in new combinations of paternal and maternal alleles in each chromatids Independent assortment of homologous chromosomes during metaphase I results in random distribution of paternal and maternal chromosome in each gamete Sexual reproduction - Random fusion of 2 haploid gametes to form a diploid zygote restores the diploid number - genetic variation due to random fusion of gametes from each individual + random mating between individuals in a population

Question 29

Effect of environment on Phenotype

Answer 29

``` Temperature Elevation Soil Acidity Diet Light ```

Question 30

Effect of Temperature

Answer 30

Coat colour in Himalayan Rabbits White body with black ears, nose, feet and tail All Himalayan rabbits are homozygous for ch allele of tyrosinase gene, which codes for a heat sensitive form of enzyme tyrosinase, which is needed for melanin production resulting in black fur Heat sensitive tyrosinase active only when air T is low - so extremeties or underneath ice pack (shave, put ice pack - fur grows back black)

Question 31

Effect of Diet

Answer 31

Honey bees Queen and workers are females and develop from fertilised diploid eggs Same genetic material but phenotypically different All larvae are initially fed with royal jelly, but larvae to be worker bees are switched to a diet containing honey and pollen, while larvae destined to be the queen are fed with royal jelly High protein content in royal jelly stimulates formation and maturation of female reproductive system

Question 32

Gene Interaction

Answer 32

Idea that 2 or more genes influence one particular character Commonly, various gene products function in a metabolic pathway that contributes to development of one particular phenotypes

Question 33

Non-epistatic gene interaction

Answer 33

Two independently assorting genes (unlinked) may interact to influence a single character Similar to dihybrid inheritance for ratio (9:3:3:1 for interbreeding of hets at both positions)

Question 34

Epistatic gene interaction

Answer 34

Expression of an allele of 1 gene suppress / inhibit expression of alleles at a different gene Not complete dominance - mask at same gene locus

Question 35

Epistatic gene

Answer 35

Able to suppress / inhibit the effect of a gene at a different locus Can be recessive or dominant Suppressed gene is called hypostatic gene

Question 36

Recessive Epistasis

Answer 36

2 recessive alleles at epistatic gene locus (e.g. preventing pigment deposition) will suppress / inhibit effect of either allele of hypostatic gene at different locus (9:3:4 for interbreeding of hets at both positions)

Question 37

Dominant Epistasis

Answer 37

One dominant allele at epistatic gene locus will suppress/ inhibit effect of both alleles of hypostatic gene at different locus (12:3:1 for cross of hets at both positions)

Question 38

Duplicate Recessive Epistasis

Answer 38

2 recessive alleles at either gene loci will suppress / inhibit the effect of dominant allele at other gene locus 9:7

Question 39

Duplicate interaction

Answer 39

9:6:1

Question 40

Duplicate dominant epistasis

Answer 40

15:1

Question 41

Dominant and recessive epistasis in double heterozygous cross

Answer 41

13:3 A epistatic to B and b b epistatic to A and a

Question 42

Discontinuous Variation (Variation in characters of type)

Answer 42

Phenotypes are definite and clear cut - discrete phenotypic classes, no intermediates Variation controlled by single or few genes Little or no environmental effect Bar graphs Qualitative : Count

Question 43

Continuous Variation (Variation in characters of degree)

Answer 43

Range of phenotypes / intermediates observed Variation controlled by multiple additive genes (known as polygenic inheritance) Cumulative effect of varying environmental factors acting on different genotype - degree of expression of genotype (determined at fert) hinges on environmental factors Normal distribution curve, histograms Quantitive: Estimates of population parameters such as mean and standard deviation

Question 44

Generating new alleles

Answer 44

Mutation (change in nucleotide sequence - gene mutation, change in structure or number of chromosomes) Only mutations occurring during gamete formation can e inherited

Question 45

Chi-squared test

Answer 45

Discontinuous / discrete variables Calc > crit Probability that chance alone is the reason for difference between observed and expected results / ratio is less than 5% Significant deviation Reject null hypothesis, differences are not due to chance Crit > calc Probability that chance alone is the reason for difference... more than 5% Deviation not significant Expected and observed results are in line - accept null hypothesis (any differences are due to chance)