1. Genetic crosses: Rules of the game

2. Making melanin pigment
The TYR gene is just one of many genes on the human number-11 chromosome.
The TYR gene encodes a protein that functions as the enzyme, tyrosinase. This enzyme catalyses a step in the pathway that produces the pigment, melanin.
Melanin pigment is seen in the hair, the skin and the irises of a person’s eyes. Melanin pigment is present not only in people, but also in other vertebrate species in their skin, eyes and fur (in the case of mammals), feathers (in the case of birds) and scales (in the case of reptiles).
Melanin pigmentation is produced in a multi-step pathway in special cells known as melanocytes.
The enzyme tyrosinase catalyses one step in the pathway that produces melanin. Without a functioning tyrosinase enzyme, melanin production cannot occur and this results in a condition known as albinism.

3. The TYR gene in humans has two common alleles:
allele A that produces normal tyrosinase enzyme, resulting in normal pigmentation
allele a that produces a faulty protein that cannot act as the tyrosinase enzyme, resulting in a lack of pigment (albinism).
Persons with genotypes AA or Aa have normal pigmentation. In contrast, persons with the aa genotype lack a functioning tyrosinase enzyme and so have albinism. From this, we can conclude that normal pigmentation phenotype is dominant to albinism

4. Rules of the genetic game
A cross involving the alleles of the TYR gene is a monohybrid cross because it involves the alleles of just one gene at a time.
When the alleles of two genes are involved, the cross is termed a dihybrid cross. A monogenic cross involves the segregation of alleles of the same gene into separate gametes. This segregation, or separation, occurs when homologous chromosomes disjoin at anaphase 1 of meiosis.

5. Tracey and John are planning their next pregnancy
Tracey and John are planning their next pregnancy. One of their first-born non- identical twin children, Fiona, has the condition of albinism and the parents want to know about the chance of this condition appearing in their next child.
Both parents are carriers of albinism and have the genotype Aa.

6. Monohybrid crosses: pigment or not?
For the TYR gene on the number-11 chromosome, which controls pigment production, the cross can be seen 
During meiosis, the pair of number-11 chromosomes disjoin, carrying the alleles to different gametes. Tracey’s eggs have either the A allele or the a allele. This also applies to the sperm cells produced by John.
This separation of the alleles of one gene into different gametes that occurs during meiosis is known as the segregation of alleles. For each parent, the chance of a gamete with A is 1 in 2 and the chance of a gamete with a is also 1 in 2.
These probabilities can also be incorporated into a Punnett square. The AA and the Aa genotypes both result in normal pigmentation. The aa genotype causes albinism.

7. A Punnett square shows the chance of each possible outcome, not what will happen.
So, Tracey and John asked,
‘What is the chance that our next child will have albinism?’
The answer to their question is 1 in 4, or ¼. The chance that their next child will have normal pigmentation is ¾.
If the next child has normal pigmentation, what is the chance that this child will be a heterozygous carrier of albinism?
There are three ways a child with normal pigmentation can result, so the chance that this child will be a heterozygous Aa carrier of albinism is 2 in 3.

8. Monohybrid cross: abo blood type
The four blood groups in the ABO blood system are A, B, AB and O.
These different blood group phenotypes are controlled by the ABO gene on the number-9 chromosome.
Each phenotype is determined by the presence or absence of specific proteins, known as antigens, on the plasma membrane of the red blood cells.
Antigens present on red blood cells
ABO blood group
Frequency in Australian population*
antigen A A
38%
antigen B B
10%
antigens A and B AB
 3%
neither antigen O
49%
* based on Australian Red Cross data

9. Multiple alleles for one gene
Blood type is an example of a multiallelic system where by three or more alleles of a gene exist among the members of a population.
Blood groups are therefore said to be a multialleic trait.
The fact that there are more than two alleles responsible for determining the blood group makes no difference to their transmission, which takes place in normal Mendelian fashion.

10. Tracey and John are both blood group B and their twins are both blood group O. This means that both parents have the heterozygous genotype IBi, while the twins have the genotype ii. The cross between Tracey and John for the ABO gene is:

11. The chance that Tracey and John’s next child will be blood group B is 3 in 4 and the chance that it will be blood group O is 1 in 4.
Remember that ratios, such as 3 to 1, identify the chance or the probability of a particular outcome occurring; they do not identify a certain outcome.

12. Variations of these crosses
Co-dominance
Incomplete dominance
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Lethal genes

13. Lethal genes
Some genotypes can result in death early in embryonic development. Offspring with such lethal genotypes do not develop and so do not appear among the offspring in a cross. An example of this is fur colour in mice. Read page 305 – 306
Gametes Y y YY Yy yy
Parental Phenotype
Yellow x Yellow
Parental Genotype
Yy x Yy
Possible Gametes
YY and Yy and yy

14. Genotypic and phenotypic ratios
When you finish a Punnett square you are often required to provide a summary of the results e.g. 1 in 4 will be recessive for the trait, 50% will be brown etc.
You may be asked also to represent the results as a ratio.
If we were to look at the growth of daffodils and used the letter G to represent growth. The ratios would be:
1 GG : 2 Gg : 1 gg (Remember this is complete dominance!)
3 Tall : 1 Short

15. Monogenic vs. polygenic traits
Mono = one Genic = gene
Traits that are controlled by a single gene.
Typically show discontinuous variation; meaning a small number of phenotypes often just two are produced
Polygenic
Poly = many Genic = gene
Traits that are controlled by many genes
Typically show continuous variation; where a range of phenotypes are expressed
Typically quantitative and are often described in numerical values, such as in height 175cm, 6ft 2in

16. Polygenic inheritance
Polygenic inheritance is the transmission of a characteristic that is controlled by two or more genes (polygenes), for example human skin colour and height.
Unlike Mendelian inheritance where plants were either tall or short, humans have a range of heights.
In addition to involving multiple genes, polygenic inheritance also looks at the role of environment in someone's development.
When a trait shows a range of phenotypes, this is called continuous variation.

17. Polygenes and human height
Adult human height shows a large range of phenotypes.
Adult height can be affected by illnesses in childhood, diet, genetics
For this polygenic trait in humans it is reasonable to predict that the distributions of height in a large sample of adults will show a bell-shaped distribution.
What could influence skin colour?

18. True or false? Polygenic traits typically show discontinuous variation
The greater the number of polygenes, the greater the number of possible phenotypic classes
Polygenic traits measured over a large sample of people would be expected to show a bell-curve type distribution.

19. Test crosses