1. Inherited Change Part II

2. Genetics
Genetics is the branch of Biology that studies heredity. Heredity is determined by the genes that are passed on from parent to daughter cells.
Genes are pieces of DNA that code for the production of a polypeptide molecule. This code is embedded in the sequence of bases within the DNA.
A triplet of bases stands for one amino acid in the protein that will be constructed on the ribosomes in the cell.
One chromosome contains enough DNA to code for many polypeptides. If chromosomes contain DNA that make the same polypeptides, they are said to be homologous.
This means that, in a diploid
cell, there are 2 copies of
each gene. These copies of
each gene lie in the same
locus on both homologous
chromosomes.

3. Alleles Each species has a specific number of chromosomes.
Human cells contain 46 chromosomes, 2 of each 23 types.
Each type is assigned a specific number and contains its own particular genes.
In order to have the total 46 chromosomes, a cell obtains 23 from the mother (maternal chromosomes) and 23 from the father (paternal chromosomes).
There are different forms or varieties of
genes. These are known as alleles.

4. Genotype and Phenotype
Most genes have several different alleles. To make things simple, the different alleles of a gene can be represented by symbols.
HbA = the allele for the normal βpolypetide
HbS = the allele for the sickle cell polypeptide
NOTE: The Hb stands for the locus of the haemoglobin gene. The superscripts stands for the particular allele of the gene.
The alleles that an organism has composes its genotype.
When the 2 alleles of a gene are the same, it is said to be homozygous.
When the 2 alleles of a gene are different, it is said to be heterozygous.
HbAHbA or HbAHbS or HbSHbS
The observable characteristic of an organism is its phenotype.
HbAHbA = normal
HbAHbS = carrier / normal but with the trait (sickle cell trait)
HbSHbS = sickle cell anemia

5. Inheriting Genes HbAHbA HbAHbS HbSHbS
Gametes, during sexual reproduction, are created from diploid body cells. Therefore, each gamete will carry one chromosome of each pair. This means that each gamete has only one copy of each gene.
Depending on the genotype of the individual, and the gametes that can be produced by the individual, the possible genotypes of the offspring can be predicted.
HbA HbS
HbA HbAHbS  2/4 = 0.50%
HbAHbA  1/4 = 0.25%
HbS HbSHbS  1/4 = 0.25%
NOTE: Please understand that these are only probabilities!!
HbAHbA
HbAHbS
HbSHbS

6. Genetic Diagrams
The standard way of showing the genotypes of offspring that might be expected from 2 parents is via a genetic diagram.
A genetic diagram will always include:
The parental phenotypes
The parental genotypes
The possible gametes for both parents
The possible offspring genotypes with the probabilities for each
The possible offspring phenotypes with the probabilities for each
The punnett square that predicted the above for the offspring for the chosen parents.
EXAMPLE: (Codominance)
CR – Red flowers CRCR – Red flowers
CW – White flowers CRCW – Pink flowers
CWCW – White flowers

7. Dominance
There are instances where both of the alleles in an organism have an affect on the phenotype of the organism. This means that the alleles are said to be codominant.
When dealing with this scenario, you write the genotype of the organism with a capital letter for the gene and the superscript for the corresponding allele. (HN or CW)
However, there are instances where only one allele has an effect on the phenotype of the organism. This means that the allele is dominant. If the allele has no effect on the phenotype of the organism, the allele is recessive. (Aa, AA, or aa)
When dealing with this scenario, you can assign a capital letter to the dominant allele and a lower case letter to the recessive allele. However, make sure that you can differentiate between the capital and the lower case of the symbol that you chose.

8. Test Crosses
Unfortunately, when alleles show dominance, it is not possible to be able to tell the genotype of the organism with a dominant allele just by looking at it.
In order to determine the actual genotype of the organism, you need to perform a test cross. A test cross involves crossing an organism showing the dominant trait with an organism that is homozygous recessive.
EXAMPLE:
A – purple stem Aa – purple stem
a – green stem AA – purple stem
aa – green stem

9. Multiple Alleles
Most genes have more than 2 possible alleles. A gene in this situation is known to have multiple alleles. Blood groups are a perfect example of this scenario. Blood groups are composed of 3 alleles.
Two of the alleles, IA and IB, are codominant.
The other allele, IO, is recessive.
Genotypes
Phenotypes
IAIA or IAIO A
IBIB or IBIO B
IAIB AB
IOIO O

10. Sex Inheritance and Linkage
Sex chromosomes in a cell are not always alike because the do not always have the same genes in the same position. This is due to the fact that there are 2 types of sex chromosomes, the X and the Y chromosomes.
The X chromosome is much larger and carriers more genes than the Y chromosome.
The genes that are carried by the female sex chromosome are said to be sex-linked. A sex-linked gene is one that is found on part of the X chromosome that is not matched by the Y chromosome.
When dealing with this situation, sex-linked genes are represented by symbols that show that they are found on the X chromosome.
EXAMPLE:
H – normal Factor VIII
h – lack of factor VIII

11. Dihybrid Crosses
Dihybrid crosses involve the inheritance of 2 genes at once.
When cells undergo meiosis to produce gametes, the pairs of homologous chromosomes line up independently of each other during metaphase I.
If many cells undergo division, it is likely that the chromosomes in roughly half of the cells will line up one way and the other half will line up the other way. This way you can predict that the gametes formed will occur in approximately equal numbers.
EXAMPLE:
Gene for stem color Gene for leaf shape
A – allele for purple stem D – allele for cut leaves (jagged)
a – allele for green stem d – allele for potato leaves (smooth)