Biology- Chromosomes and Genetics

There are 23 matched pairs of chromosomes in every human body cell. The 23rd pair are labelled the ** and XY. They're the two chromosomes that decide whether you are a male or female.

MEN: XY chromosome, The Y chromosome causes male characteristics.

WOMEN: ** chromosome, The combination of ** allows female characteristics to develop.

When making sperm, X and Y chromosomes are drawn apart in the first division of meiosis. There's a 50% chance it gets a X chromosome and a 50% chance it gets a Y chromosome.

In an egg cell, theres only one X chromosome. The original cell has two.

GENETIC DIAGRAMS= show the possible combinations of Gametes.

To find the probability of getting either a girl or a boy, you can draw a genetic diagram.

Gregor Mendel was an Austarlian monk. On his garden plot at the monastery, Mendel noted how characteristics in plants were passed on from one generation to the next.The results of Mendels experiments were published in 1866 and eventually became the foundation of modern genetics.

Mendel had showed that the height characteristic in pea plants was determined by separately inherited "hereditary units" passed on from each parent. Ratios of tall and dwarf plants in the offspring showed that the unit for tall plants,T, was dominant over the unit for dwarf plants,t.

MENDELS THREE IMPORTANT CONCLUSIONS:

• Characteristics in plants are determined by "hereditary units"
• Hereditary Units are passed on from both parents, one unit from each.
• Hereditary units can be dominant or recessive- if an individual has both dominant and the recessive unit for a charcteistic, the dominant characteristic will be expressed.

"hereditary units" are genes. In Mendels times there was no knowledge on genes so his work wasnt appreciated untill after his death.

Genetic Diagrams show the possible genes of offspring.

Alleles are different versions of the same gene. Gametes only have one allele, but all other cells in an organism have two. If an organism has two alleles for a particular gene the same, then it's HOMOZYGOUS. If its two alleles for a particular gene are different, then it's HETEROZYGOUS.

If the two alleles are different, only one can determine what characteristic is present. The allele for the characteristic that's shown is called the dominant allele. The other one is called the recessive.

For an organism to display a recessive characteristic, both its alleles must be recessive. But to display a dominant characteristic the organism can be either CC or Cc, because the dominant allele overrules the recessive one if the plant/animal/organism is heterozygous.

GENOTYPE= what alleles you have

PHENOTYPE= the actual characteristic

Cystic Fibrosis- a genetic disorder of cell membranes caused by recessive allele.People with one copy of the allele won't have the disorder- known as carriers.Polydactyly is caused by a dominant allele. Parent that has the defective allele will be a sufferer too.

During in vitro fertilisation(IVF),embryos are fertilised in a lab and then implanted into the mother's womb. More than one egg is fertilised for a higher success rate. Before being implanted, it's possible to remove a cell from each embryo and analyse its genes for genetic disorders. Embryos with good alleles can be implanted, bad ones are destroyed.

AGAINST EMBRYONIC SCREENING:

• Implying people with genetic problems are 'undesireable'
• Expensive

FOR EMBRYONIC SCREENING:

• Stop people suffering
• Saves to Government money as they don't have to sort disorders.