Genetics and Inheritance Flashcards
1
Q
Traits and Genes
A
- Various hereditary characteristics or TRAITS are controlled by GENES.
2
Q
Alleles
A
- Each gene has at least two (sometimes more) alternative forms of the gene called ALLELES.
- Alleles are represented by capital and lower case letters.
- Upper case letters are DOMINANT
- Lower case letters are RECESSIVE
- Dominant alleles mask the appearance of recessive alleles.
3
Q
Combination of Alleles
A
- For any one characteristic there may be 3 alternative combinations of alleles: GG, Gg, gg.
- These are referred to as GENOTYPES
- If the two alleles are the same they are called HOMOZYGOUS (or pure)
- If the two alleles are different they are called HETEROZYGOUS (or hybrid)
- The physical appearance of what these traits look like is called the PHENOTYPE.
4
Q
Punnet Squares
A
- Punnet squares allow us to calculate the frequency of a genotype and its appearance (phenotype) for an inherited trait.
- Through this method we are able to predict the probability of many inherited conditions.
- The offspring are referred to as the First filial generation (F1)
- If the first filial generations self-pollinated (interbred) then the second generation of offspring are called the Second filial generation (F2)
5
Q
Drawing a Punnet Square
A
Drawing a punnett square example:
1. Start off with the parent genotypes:
* Mother – GG (green) Father – gg (yellow)
* In MONOHYBRID CROSSES, only one pair of contrasting pure-breeding characteristics are studied (eg. Yellow pod and green pod colour).
- Draw the punnett square table, then enter and determine the genotypes of offspring by crossing parent genotypes:
G G
G Gg Gg
g Gg Gg - Determine the phenotypes and genotypes of offspring:
* Phenotypes: 100% Green
* Genotypes: Gg 100%
6
Q
Incomplete Dominance
A
- Incomplete Dominance is a form of inheritance in which one allele for a specific trait is not completely dominant over the other allele
- This results in the heterozygote having a combined (blended) phenotype
- Incomplete alleles are represented by upper case letters (e.g., RW)
- For example: Snapdragon flowers
- The genotype for the red flowers is RR
- The genotype for the white flowers is WW
- When a Red flower is bred with a white flower the resultant genotype is RW
- The Phenotype for the heterozygous genotype RW is a blend of the RED and white which makes Pink
7
Q
Co-Dominance
A
- Co-dominance is a form of inheritance in which both alleles for a specific trait present equally in the phenotype
- The results in the heterozygote having the characteristics of both parents (both traits appear together at the same time – they CO-exist)
- Co-dominant alleles are also represented by two upper-case letters (e.g. MN)
- For example: Roan cattle
- The genotype for the red
8
Q
Sex Determination
A
- There are 23 pairs of chromosomes in the human genome
- The first 22 pairs are called autosomal chromosomes (or autosomes)
The 23rd pair are the sex chromosomes which determine the gender of an individual - There are 2 different sex chromosomes, X and Y
- The male genotype is XY, and the female genotype is XX
- The ova produced by a woman will always carry the X chromosome
- The male sperm can carry the X or the Y, and therefore determines the sex of the offspringX X
X XX XX
Y XY XY
9
Q
Sex Linked Characteristics
A
- The X chromosome carriers’ genes for traits just like any other chromosome, we call these Sex-linked or X-linked
- X-linked – traits carried on the X chromosome
- For X-linkage the X and Y chromosomes must be shown, with the alleles represented by either upper- or lower-case letters in superscript (e.g., XHXh or XhY)
- In a male, some of the genes on the X chromosomes have no corresponding allele on the Y chromosome
- So, males only have 1 allele for some characteristics instead of a pair
- Therefore, any allele on their one X chromosome will be expressed, whether it is dominant or recessive
- Females have two X chromosomes so they will have two alleles as per usual.
- These traits are always more common in males than females
- This is because they are recessive traits, so for a male to be affected his one X chromosome must have the recessive allele (XhY), however for a female to be affected she must have 2 recessive alleles
10
Q
Calculating Probability
A
- Percentages can be written as a probability.
- Eg. 50% = 0.5 probability
- To calculate the probability of producing a female who also has the trait, the two probabilities must be multiplied.
- Eg. P = 0.5 x 0.5 = 0.25
11
Q
Variation
A
- Variation refers to the differences between members of a species.
- Sources of variation include:
- Sexual reproduction
- Random fertilisation
- Random assortment of chromosomes
- Crossing over
- Non-disjunction
- Environment
12
Q
Sexual Reproduction
A
- Mother and Father will each contribute 50% of the Baby’s DNA. Therefore having a mix of each parents features and their own unique genome.
13
Q
Random Assortment (Variation)
A
- Fertilisation occurs when a sperm fuses with an egg.
- Every egg and sperm cell carries different genetic material.
- Fertilisation is completely random and there is no way of telling which particular sperm from the male parent will fertilize which egg from the female parent.
- In addition, each sperm has a different survival rate in the female’s reproductive tract, so it completely up to chance which survive.
14
Q
Mendel’s 1st Law: Principle of segregation (Random Assortment of Chromosomes during Meiosis)
A
- Separation of members of a gene pair (2 alleles) occurs during gamete formation and two gametes randomly unite at fertilization.
- For each gene the offspring will get one allele from each parent giving them the 2 alleles required.
15
Q
Mendel’s 2nd Law: Principle of independent assortment (Random Assortment of Chromosomes during Meiosis)
A
- During gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair.
- That is for example the alleles that control eye colour are completely separate from those that control hair colour.
