Genes and inheritance Flashcards
Define a gene
gene as a unit of inheritance
distinguish gene and allele
Gene: a length of DNA that codes for a polypeptide, with a specific sequence of nucleotides as part of a DNA molecule.
Allele: variant forms of genes (1 or more alternative sequences of a single gene locus)
Locus
position on chromosomes where a gene occurs.
Dominant, recessive
Dominant: an allele with a phenotype that is expressed even when present with an allele that is recessive to it
Recessive: an allele with a phenotype that is not expressed when present with an allele that is dominant to it.
Homozygous/heterozygous
Homozygous: A genotype in which the 2 alleles of a gene are the same
Heterozygous: …different
Phenotype
the way in which the genotype of an organism is expressed
Genotype
the alleles that an organism carries, present in every cell.
Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny.
The probability calculated will apply to each individual independently, so we may not get expected ratio, especially with a small sample size of progeny.
(must be large: 4000)
Explain how genotype is linked to phenotype
Each allele from each parent has an equal chance of being passed onto offspring. 2 alleles of genes in each cell form the genotype of an organism.
One allele is dominant over the other allele, and expresses itself in both homozygous and heterozygous conditions, expressing the organism’s phenotype.
carrier
A carrier is an individual who possesses the recessive allele but appear normal phenotypically.
how genes are inherited from one generation to the next generation via the gametes.
In sexual reproduction, haploid gametes are formed by meiosis
At fertilisation, male and female gametes fuse to form a zygote
Zygote, a diploid cell, has 2 sets of chromosomes, one homologous pair from each parent
2 alleles of each gene present in the new individual
Use genetic diagrams to solve problems..
Key: Let N be dominant trait
Let n be recessive trait
Parental/F(Filial)1 generation:
Phenotype: normal x disease
Genotype: Nn x nn
Gametes(circled): N n x n n
F1/F2 generation:
Normal (Nn) D gametes N n (circled) I (random S fertilisation) E n Nn nn A S n Nn nn E (circled) (Nn)
F1/F2 generation:
Genotypic ratio: 1Nn: 1nn
Phenotypic ratio: 1normal: 1 disease
NOTE: Repeat EXACTLY what qn end says.
expected ratios
Simple crosses have expected ratios of genotype: 3:1 and phenotype: 1:2:1
Monohybrid cross
investigation of single contrasting characteristic.
Test cross and result
breeding mystery individual with homozygous recessive individual
If there are any recessive offspring, mystery parent is heterozygous
Mono/di-hybrid Test cross
Monohybrid test cross: 1:1
Dihybrid test cross: 1:1:1:1
Law of segregation
During meiosis, pairs of alleles separate so that each cell has one allele of a pair. Allele pairs separate independently during the formation of gametes.
Law of independent assortment
During meiosis, the separation of one pair of alleles is independent of the separation of another pair of alleles.
Calculate Chance: multiple offspring produced
independent chance: try 2 times of ¼ chance= ¼ *¼ = 1/16 chance
Pedigree
family tree that enables us to trace the transfer of a specific genetic trait through several generations
Sex, blood type and colour genotypes
Sex: XY (male), XX (female) X^H,X^h
Blood type: I^A,I^B, i
Colour: C^RC^W
Codominance, Incomplete dominance, Multiple alleles and examples
Codominance: alleles that are both expressed in a heterozygous organism (IAIB)
Incomplete dominance: phenotype of F1 hybrid is between the phenotypes of the 2 parental varieties (Colours(pink): CRCW)
Multiple alleles: 3 or more alleles occupy the same locus. (IA,IB, i)
describe Sex-linked inheritance
Due to genes present on the sex chromosomes
Autosomal chromosome
non sex chromosome
How pedigree shows it is Sex-linked?
Only males are affected and females are carriers.
Why Only males are affected and females are carriers.
(disease) caused by recessive allele linked to X chromosome. Male descendants only carry 1 X chromosome, increasing their chances of inheriting the disease from their ancestors. Females have 2 X chromosomes, so a single recessive allele on the X chromosome will not result in disease.
Codominance:
For 2 parents with different genotypes to produce (hetero/homo) dominant offspring:
1 parent must be heterozygous to pass on recessive allele (i) to offspring (for the homozygous parent to pass on dominant allele to have single dominant offspring).
Codminance:
For recessive offspring:
Both parents must have recessive alleles to pass in in gametes to produce recessive offspring
Discontinuous variation:
one or more discrete (pheno/geno) types with no intermediate forms, genetically determined, not affected by environmental factors
Continuous variation
environmental and genetic factors
