B7 Inheritance Flashcards
Heredity
Transmission of genetic characteristics (genes stored in chromosomes) from 1 generation to the next and the effects of this transmission
Variation
Recognisable differences between individuals of the same species and between parents and offsprings
Gene
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
Locus
Specific location of a gene on a chromosome
Allele
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
Genotype
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
Phenotype
Physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment
Wild-type
Most common allele / phenotype in nature (vs mutant allele / phenotype)
Homozygous
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
Heterozygous
Alleles of a gene pair in diploid condition are different
Organism known as heterozygote
Dominant alleles
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
Recessive alleles
Produce effects only in homozygous condition
Homozygous recessive
True-breeding
Organism is homozygous, all offspring of the same phenotype
Carriers
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
Link between genotype and phenotype
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
Mendel’s First Law of Segregation
During formation of gametes, paired alleles segregate randomly so each gamete receives one of the other with equal likelihood.
Incomplete Dominance
A condition where neither of the 2 alleles are completely dominant to the other, so heterozygote has phenotype which is intermediate
1:2:1
Codominance
Both alleles equally expressed in phenotype of heterozygote
Heterozygote simultaneously expresses phenotypes of both types of homozygotes
Multiple alleles
Gene controlling characteristic has 3 or more alleles
Mendel’s Second Law of Segregation
Segregation of one pair of alleles is independent of segregation of other pairs
Sex Linkage
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)
Linked genes
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
Complete Linkage
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)
Incomplete Linkage
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
Heterozygotes - Linked genes arrangements
Coupling - 2 dom alleles on one chromosome, 2 rec alleles on one chromosome
Repulsion - dom + rec on each chromosome
Distance between genes based on proportion of recombinants
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
Phenotypic Variance
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.
Ways of introducing variation in genotypes - new combination of existing alleles, nOT new alleles!
(Asexual organisms - Variation almost always from environmental influences)
- 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 - Chromosomal Mutation
Change in structure or number of chromosomes - 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
Effect of environment on Phenotype
Temperature Elevation Soil Acidity Diet Light
Effect of Temperature
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)
Effect of Diet
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
Gene Interaction
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
Non-epistatic gene interaction
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)
Epistatic gene interaction
Expression of an allele of 1 gene suppress / inhibit expression of alleles at a different gene
Not complete dominance - mask at same gene locus
Epistatic gene
Able to suppress / inhibit the effect of a gene at a different locus
Can be recessive or dominant
Suppressed gene is called hypostatic gene
Recessive Epistasis
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)
Dominant Epistasis
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)
Duplicate Recessive Epistasis
2 recessive alleles at either gene loci will suppress / inhibit the effect of dominant allele at other gene locus
9:7
Duplicate interaction
9:6:1
Duplicate dominant epistasis
15:1
Dominant and recessive epistasis in double heterozygous cross
13:3
A epistatic to B and b
b epistatic to A and a
Discontinuous Variation (Variation in characters of type)
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
Continuous Variation (Variation in characters of degree)
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
Generating new alleles
Mutation (change in nucleotide sequence - gene mutation, change in structure or number of chromosomes)
Only mutations occurring during gamete formation can e inherited
Chi-squared test
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)
