Alleles (2) Flashcards
What are alleles?
Alleles are genes responsible for controlling different versions of a trait/characteristic found in the same locus (position) on homologous chromosomes.
Eg. for the gene determining coat colour in cattle, one allele could determine a ___ coat (B) and another allele a ___ coat (b).
Black
White
Define allele.
one of two or more forms of a gene (from the word, allelomorph, meaning ‘alternative form’)
How are alleles represented?
When describing characteristics, alleles are represented by means of letters. Very often the capital letter denotes the dominant allele and a lower case of the same letter represents the recessive allele.
How are alleles passed from parent to offspring?
Alleles are passed from parents to offspring by way of chromosomes in the gametes that are made by the process of meiosis in the sex organs.
Define gamete.
a reproductive cell/sex cell.
Normal body cells (somatic cells) are diploid (2n) as they have:
- a pair of homologous chromosomes
- two alleles which may be the same or different.
Sex cells (gametes) are haploid (n) as they have:
- one of a pair of homologous chromosomes
- one of each pair of alleles.
What happens to the gametes?
During fertilization a male gamete, with its alleles, fuses with a female gamete, with its alleles, to form a diploid zygote. This divides and divides by mitosis to form an entirely new organism made up of cells, each with the same set of chromosomes and alleles as in the zygote.
What is a genotype?
A genotype is made up of all the genes an organism carries on its chromosomes which it has inherited from its parents.
What is a phenotype?
A phenotype is the physical appearance of an organism, such as tallness, hair colour. It is partly programmed by genes, its genotype, but also shaped by external factors such as exercise, diet and environment.
What happens after the fertilisation of the zygote?
After fertilization, the zygote, and all the cells that develop from it, will have two alleles for each gene, one from each parent. These will be in the locus of each chromosome of the same homologous pair.
What happens if the pair of alleles at a locus are the same?
If the pair of alleles at a locus are the same, the organism is homozygous for that particular trait, e.g. in our example, both alleles will determine a black coat in cattle (BB). homo = same
What happens if the pair of alleles at a locus are different?
If the pair of alleles at a locus are different, the organism is heterozygous (a hybrid) for that particular trait, e.g. one allele determining a black coat and the other a white coat in cattle (Bb). hetero = different
In heterozygous pairing, one allele of a pair may be
- dominant, in that this trait is expressed in the offspring, e.g. black coat.
- recessive, in that the trait is suppressed in the presence of the dominant allele and not expressed in the offspring.
What is a monohybrid cross?
A monohybrid cross is a cross between parents with different alleles for a single gene.
What do genetic diagrams show?
Genetic diagrams show how characteristics are inherited. They show the genotype and the phenotype of a cross between two parents and help us understand why the offspring look the way they do.
Generations are shown as follows: (3)
P1 - parent generation
F1 - first filial generation of offspring
F2 - second filial generation of offspring
Alleles can be shown as ___ and ____ letters. Very often the capital letter denotes the dominant allele and a lower case of the same letter represents the recessive allele.
capital
lowercase
As there are two alleles for each characteristic, one on each chromosome of a homologous pair, two letters are written, eg. ___.
Tt
What is a punnet sqaure?
A punnett square is an easy way to represent a cross between two organisms for any number of characteristics for which the parental genotypes are known. It predicts the probability of the offspring’s genotype and phenotype and is the basic tool used for Mendelian genetics.
How can the the characteristics of the offspring be determined?
The characteristics of the offspring can be determined by combining the allele of each gamete with the allele that intersects in each of the squares.
The phenotype is determined by the alleles in each offspring square: an uppercase letter represents a ____ allele.
dominant
When will the recessive allele be expressed?
The recessive allele will only be expressed in the offspring when it is homozygous, i.e. when no dominant allele is present.
