Genetics Flashcards
Father of modern genetics
Gregor Mendel
discovered using his experiments involving principles of heredity by breeding garden pea plants (Pisum Sativum)
Character
A heritable feature which varies among organisms of a population
Traits
Heritable variants of a character in an organism
Phenotype
Observable traits of an organism
Gene
Specific DNA sequence residing on a locus of an particular chromosome and involves in the development of 1 or more traits by coding to specific proteins or peptides
Allels
Alternative versions of genes residing on the same locus but on different chromosomes (homologous chromosomes). Alleles vary in the sequence of nucleotides thus changing the phenotype of the organism.
Genotype
The genetic makeup or set of allels of an organisms.
Phenotyoe is a result of the interaction between genotype with the environment
Contrasting traits
2 distinct contrasting phenotypic forms of a character are called contrasting traits.
Mendel studied inheritance of 14 contrasting traits of 7 characters of garden pea plants.
Pure lines
Uniform lines produced from self-fertilisation of pure breeding varieties over many generations are called pure lines.
Hybridisation
Mating or crossing of 2 pure-breeding varieties with contrasting traits is called hybridisation.
e.g: pure breeding tall plants crossed with pure breeding short plants
Monohybrid
An organism that is heterozygous resulting from a cross between parents having homozygous conditions for different alleles of specific gene is referred to as monohybrid.
Monohybrid cross
Breeding experiment conducted between 2 organisms with heterozygous condition for a specific character is referred to as a monohybrid cross. (cross between 2 monohybrids)
Dihybrid
An organism that is heterozygous with respect to 2 genes of interest resulting from a cross between parents having homozygous conditions for different alleles of 2 specific genes is referred to as a dihybrid.
Dihybrid cross
Cross between the homozygous organisms having contrasting traits for 2 specific characters.
Back cross
Crossing of offsprings with a parental organism is called a back cross.
Back crosses are performed to transfer more parental characteristics to offspring.
Test cross
Breeding an organism having unknown genotype for a specific dominant trait with an organism having homozygous recessive condition for the same specific trait is called a test cross.
Mendelian ratio for monohybrid crosses
3:1
Mendelian ratio for dihybrid crosses
9:3:3:1
Explain mendel’s 1st law of inheritance
The law of segregation
Each heritable character is determined by 2 heritable factors which are known as allels.
During formation of gametes alleles for a specific character gets separated and gets into each of the gamete formed.
Punnett square
Graphical representation of the possible genotypes of an offspring arising from a particular cross or breeding event.
Dependant assortment of alleles
The 2 characters could be transmitted from parents to offspring as a package.
Mendle’s 2nd law of inheritance
The law of independent assortment
Alleles separate and pair up independently during gamete formation
According to current genetics knowledge, independent assortment applies to 2 circumstances only
Genes located on different chromosomes (genes on non-homologous chromosomes)
Genes located far apart on the same chromosome
Success of Mendel’s experiment
He carried out 1000 of genetic crosses for any given kind( larger the sample size, closer will be resulted value from the probability predictions)
Kept accurate records of results (trace patterns which otherwise would go unnoticed)
followed each of the crosses at least unto 2 offspring generations (uncover hidden traits in F1 generation)
quantitative analysis of phenotypes of resulting offsprings
Desirable properties in garden pea plants for genetic experiments
Pea plants are available in many varieties with contrasting traits
The generation time is short
A large number of offspring is produced from each cross
Crossing between the plants could be strictly controlled
Probability meaning
Measures how likely an event is to occur out of the number of possible outcomes.
Multifactorial cross
When the pattern of inheritance of two or more characters of an organism is being traced during a genetic cross, it could be called as multifactorial cross.
According to law of segregation, a multifactorial cross can be equivalent to multiple independent monohybrid crosses occurring simultaneously.
If independent assortment of alleles occurs, the number of different types of gametes produced by a particular genotype equals
2 x power nth
n = number of heterozygous allele pairs
f independent assortment of alleles occurs during gametogenesis and if the cross is performed between the same genotype in the F1 generation(self fertilisation), the number of different types of genotypes produced by a particular genotype and the number of phenotypes produced equals
No. of genotypes produced - 3 x power nth
No. of phenotypes produced - 2 x power nth
n = number of heterozygous allele pairs
Attached or detached earlobe:
attached earlobe is a recessive trait.
Widow’s peak:
due to a dominant allele
Dimples on cheek:
Dimple is a dominant trait
When a person a smile shorter facial muscles pulls up the facial skin forming a slight depression in skin
Bent thumb (Hitchhiker’s thumb) and Straight thumb
Having the dominant ‘S’ allele would produce the dominant phenotype of straight thumb
Due to the hyper extensibility of the interphalangeal joints
Rolling or non-rolling tongue
Rolling the tongue into a tube shape is a dominant trait
Tongue’s intrinsic muscles of certain individuals allow them to roll their tongues into specific shapes
Pedigree analysis
Diagrammatic representation of the inheritance of a particular trait within a given family tree, is called pedigree chart.
Non-Mendelian inheritance
Refers to the inheritance patterns in which traits do not segregate in accordance with Mendel’s laws of inheritance.
E.g for Non-mendelian inheritance patterns
· when alleles are not completely dominant or recessive (incomplete dominance and co-
dominance),
· when a particular gene has more than two alleles (polyallelism)
· when a single gene produces multiple phenotypes (pleiotropy)
· sometimes two or more genes are involved in determining a particular phenotype (epistasis
and polygenic inheritance)
· gene linkage
· genes which are located in sex chromosomes exhibit a different pattern of inheritance.
complete dominance.
The phenomenon of dominant allele completely masking the recessive phenotype, resulting
similar phenotypes for both homozygous dominant zygote as well as heterozygous zygote is
called complete dominance.
incomplete dominance.
at the heterozygous state, the phenomenon of expressing blend phenotypes
from both alleles is called incomplete dominance.
e.g; Mirabilis jalapa (four o’ clock plants)
Polyallelism (Multiple alleles) e.g:
Polyallelism refers to the presence of multiple alleles for a single genetic locus, a phenomenon where certain traits are determined by the combination of more than two types of alleles.
e.g: there are 3 alleles called IA, IB and i for a single genetic locus which at different combinations determine ABO blood groups in humans.
Codominance
In certain traits, at heterozygote state, expression of both alleles contributes equally to the phenotype. Such phenomenon is called co-dominance. e.g: AB blood group of humans
Epistasis
Phenomena resulting from interaction between genes of different loci. The alternation in the phenotypic expression of a gene at one locus is due to the interference of another gene at different locus.
2 types namely dominant epistasis and recessive epistasis (depends on whether the epistatic allele is dominant or recessive)
Dominant epistasis
A dominant allele at a specific locus alters the expression of a gene at a different locus e.g: plumage colour of house fowls
Phenotypic ratio is 13:3
Recessive epistasis
A homozygous recessive genotype of a particular chromosomal locus alters the expression of a gene at a different locus. e.g: flower colour in sweet pea
Phenotypic ratio is 9:7
Polygenic inheritance
Inheritance of a phenotype such as quantitative characters which results from a cumulative expression of 2 or more genes is called polygenic inheritance
How to find the number of phenotypic classes of genes involved in polygenic inheritance if independent assortment of alleles occurs
2n + 1
n = number of genes involved in polygenic inheritance
Genetic linkage
Certain genes are located on the same chromosome and in close distance to each other hence they escape from crossing over and independent assortment and are inherited together
e.g: body colour and wing size of fruit fly drosophila
Which sex chromosome is bigger
X chromosome
Inability to produce normal sperm
absence of certain Y-linked genes causes inability to produce normal sperm.
X-linked recessive disorders
Red green colour blindness, Haemophilia
Haemophilia
An X-linked recessive disorder where one or more of the proteins required for blood clotting are absent. Haemophilic person run the risk of severe bleeding during injuries due to the delay in clot formation.
Pleiotropy
Expression of a single gene affects the expression of multiple traits which are not related to each other. Pleiotropic alleles are responsible for sickle-cell diseases and cystic fibrosis
Sickle-cell disease
Sickle cells may clump and clog small blood vessels causing tissue and organ damage in several body parts. This may result renal failure, heart failure and thrombosis.
Cystic fibrosis
Autosomal recessive disease condition which causes mucous to be thicker and stickier than its normal state, which results in the accumulation of mucous in pancreas, lungs, digestive tract and the reproductive tract causing lung infections, respiratory failure, poor digestion and infertility.
Thickening of mucous is due to secretion of excess chlorine from trans membrane chloride channels of the the plasma membrane as an effect of Cystic fibrosis Trans-membrane regulator protein (CTFR). Alteration of the CTFR protein is due to the mutation of the CTFR gene.
Epigenetics
Study of occurrence of phenotypes of certain characters which are controlled by factors other than their DNA sequence or genetic code. This is due to ‘switching on’ and ‘switching off’ of certain genes by modifying nucleotides of a DNA sequence by methylation and demethylation, where methyl groups are added to wild type DNA sequence or else removed from a methylated DNA sequence
Epigenetic inheritance
Epigenetics results due to either inherited signals from parents or signals arising due to the environmental factors. Inheriting epigenetic traits from parents to the children’s generation is called epigenetic inheritance.
e.g: Schizophrenia in identical twins
Hardy-Weinberg equilibrium
Used to identify whether a population is evolving respect to a particular genetic locus
population that is not evolving, allele and genotype frequencies will remain constant from generation to generation
Conditions for Hardy-Weinberg equilibrium
Absence of mutations - alter alleles modifying the gene pool
Occurrence of random mating - breeding occurs randomly without any influence on selectiveness. Mating between closely related organisms can alter allele frequency
Absence of natural selection- all organisms of the progeny are expected to survive. Variations in survival and reproduction can alter allele frequency
Size of the population is extremely large - In small populations certain genotypes may disappear due to death and infertility.
Absence of immigration or emigration - Individuals moving in and out of the population may cause appearance of new genes and disappearance of existing genes.(gene flow)
Evolution of a species
A species evolves when changes in gene frequency drives the species to a higher level of adaptation for a specific ecological niche
e.g: evolution of the peppered moth in England during time of industrialisation
Breeding
This phenomenon wherein human beings interfere in the process of reproduction to allow only selective mating to occur, so that offspring with improved characters are produced is called breeding
Major breeding techniques
Artificial selection
Inbreeding and outbreeding
Hybrid breeding
Interspecific breeding
Artificial selection
Is the process of selective breeding where plants or animals with specific traits are selected to breed so that the desired trait could be passed onto the next generation to produce a high performing variety. The main advantage is that it follows the principle of natural selection and unfavourable traits can be removed from the population effectively.
However selective breeding takes a long time to complete. e.g: in horse breeding, breeding should be continued unto 7 generations (25-50 years)
Inbreeding
Breeding between genetically similar individuals
In plant breeding, inbreeding is referred to as self-fertilisation and is used to produce uniform lines. Used in agriculture to accumulate superior genes.
In animal breeding, inbreeding refers to the breeding of closely related individuals (father and daughter, brother and sister or cousins). Through continued inbreeding harmful recessive genes can be exposed which would otherwise stay hidden in heterozygotes and can be effectively omitted from the population. However continued inbreeding reduces the genetic fitness of the progeny. (inbreeding depression)
Outbreeding
When plants or animals of different breeds (races) are mated with each other, it is known as outbreeding or cross breeding.
Hybrid breeding
When genetically unrelated pure-bred plants or animals in the same species are mated with each other it is known as hybridization or outcrossing.
Interspecific breeding
male and female organisms of two different species are mated. The progeny obtained from such a mating are usually different from both the parental species and may be fertile, partially fertile, or sterile.
natural interspecific breeding can be seen intently in plants and usually the progeny of plant interspecific breeding are physiologically successful.
Polyploidy
Refers to the presence of more than 2 complete sets of homologue chromosomes in a cell nucleus. Artificially induced in plants by the anti mitotic agent colchicine.
Plants with higher policy levels have larger organs (giga effect) and have slow growth rates which are desirable properties in ornamental plants.
Moreover due to the extra chromosomes present, certain effects of deleterious alleles are masked and results in reduced fertility. This enables the production of seedless varieties. e.g: tripod watermelon
Polyploids can be used as bridges in gene transferring as well.
