SEX DETERMINATION
How and why did you end up a girl and not a boy?
A sperm is haploid (has 23 chromosomes, not 46) - so is an egg. So when a sperm and egg fuse during fertilization, the zygote ends up with a full set - 46. The chromosomes match up with their homologous pairs. Sex is determined by the chromosomes you get in the 23rd pair. Because mum's female, she has 2 X chromosomes for her 23rd pair, but males have a different combination. To be male, you must have 1 X and 1 Y chromosome. So dad has the genotype XY.
Mum XX
The only possible chromosomes from the 23rd pair that mum has to put in her gametes (during meiosis) are X's. So no matter what, the egg she makes will have an X
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Dad XY
The possible chromosomes from the 23rd pair that dad has to put in his gametes (during meiosis) are X's AND Y's - he has one of each! So half of his sperm will carry an X chromosome and half will carry a Y.
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When fertilization happens, the zygote will definitely receive an X from mum (as mum can only give X's). It completely depends on the sperm that fertilizes the egg to which gender the offspring will be - if a sperm carrying an X fertilizes the egg, the offspring will be XX and be female. However, if the sperm is carrying the Y chromosome and fertilizes the egg then the offspring will have the genotype XY and be male. Each time fertilization happens there is a 50% chance of the offspring being male of female!
MONOHYBRID INHERITANCE
From here on, alleles are given letters. The letter is always the same (as it represents the gene) but it can be upper case or lower case (e.g. the letter E, but written as E or e). Upper case signifies dominance whereas a lower case letter is recessive. For the same gene, there is always one dominant allele and one recessive allele, at least!
New terms used to describe someone's genotype:
- Homozygous - homo meaning two of the same allele. That person's homologous chromosome pair for the gene [whatever the gene is] both contain the same allele (ie. EE or ee, not Ee).
- Heterozygous - hetero meaning two different alleles. That person's homologous chromosome pair for that gene [whatever the gene is] contain one of each allele (ie. Ee, not EE or ee).
- Dominant - When present in the genotype this allele will always show up in the phenotype. Only one is needed to 'win' and to be seen.
- Recessive - Can only show up in phenotype if there are two in the genotype. If there is ever a dominant allele and a recessive allele in a genotype, the dominant is shown in the phenotype. So 2 recessive alleles are always needed for that trait to be seen.
Monohybrid cross - A monohybrid cross is when you take two individuals and cross them (mate them), and look at the resulting offspring for one gene. We use monohybrid crosses to predict the genotype, and thus the phenotype, of the offspring that would result from a cross between those two individuals. For example, you are looking at the hair colour gene (we'll give that gene the letter B. You could give it a different letter though!).
Black hair is dominant over brown hair. Therefore we can give black hair the B and brown hair the b. B/b represents hair colour, the different cases represent dominant/recessive.
We are going to cross a homozygous dominant woman with a homozygous recessive man.
Homozygous dominant for hair colour gene = BB
Homozygous recesive for hair colour gene = bb
Black hair is dominant over brown hair. Therefore we can give black hair the B and brown hair the b. B/b represents hair colour, the different cases represent dominant/recessive.
We are going to cross a homozygous dominant woman with a homozygous recessive man.
Homozygous dominant for hair colour gene = BB
Homozygous recesive for hair colour gene = bb
Possible Genotype And Phenotype Ratios (There Are Only A Few You Can Get)
We'll do H for hairy (think back to guinea pigs) and h for hairless
GENOTYPE
100% or all HH 100% or all Hh 100% or all hh 50% HH 50% Hh or 1HH:1Hh 50% Hh 50% hh or 1Hh:1hh 25% HH 50% Hh 25% hh or 1HH:2Hh:1hh |
PHENOTYPE
100% or all Hairy 100% or all Hairy 100% or all Hairless 100% or all Hairy 50% Hairy 50% Hairless or 1 Hairy : 1 Hairless 75% Hairy 25% Hairless or 3 Hairy : 1 Hairless |
TEST CROSSES
This is one of those things you can just memorize because it will always be the same!
A test cross is a method used to find out the genotype of an individual who possesses a dominant PHENOTYPE
This is of relevance if you are wanting to create a pure-breeding pedigree (a family lineage where no alleles are hidden)
For example, HH and Hh both produce Hairy guinea pigs, but how do we know it's genotype just from looking at it? All we can see is that it is hairy!
A test cross is a method used to find out the genotype of an individual who possesses a dominant PHENOTYPE
This is of relevance if you are wanting to create a pure-breeding pedigree (a family lineage where no alleles are hidden)
For example, HH and Hh both produce Hairy guinea pigs, but how do we know it's genotype just from looking at it? All we can see is that it is hairy!
We Do a Test Cross!
By looking at this guinea pig, we know its phenotype for the 'hair' allele is hairy (H). Hairy is dominant over hairless. If there was a hairless guinea pig here instead, we would know its genotype. It would be hh. Remember any time a dominant allele is present it is expressed in the phenotype, so if the phenotype is not the dominant one, we know the genotype must contain two recessive alleles.
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All we know from looking at the guinea pig is that it is hairy, but we don't know if it is HH or Hh. To create a pure-breeding line, we want the guinea pig to be HH. To find out its genotype we must cross (mate) it with a guinea pig that is homozygous recessive for that gene (hairless - hh) and look at the offspring from that cross.