Biology - Combined Science Section 13: Reproduction. (Genetic Diseases ( …
Biology - Combined Science
Section 13: Reproduction.
Types of Reproduction
In asexual reproduction, there is no fusion of gametes and only one parent.
There is no mixing of genetic information leading to genetically identical offspring (clones).
Only mitosis is involved in asexual reproduction.
Sexual reproduction involves the joining (fusion) of male and female gametes formed by meiosis.
Meiosis leads to the formation of non-identical cells, sperm and eggs cells in animals and pollen and egg cells in flowering plants.
In sexual reproduction there is a mixing of genetic information that leads to variation in the offspring.
Fertilisation / Meiosis
Cells in the reproductive organs divide by meiosis to form the gametes (sex cells).
Body cells have two sets of chromosomes, but gametes only have one set.
In meiosis, the genetic material is copied and then the cell divides twice to form four gametes, each with a single set of chromosomes.
All gametes are genetically different from each other.
Gametes join at fertilisation to restore the normal number of chromosomes.
The new cell divides by mitosis.
The number of cells increase and as the embryo develops, the cells differentiate/
DNA & The Genome
The genome of an organism is the entire genetic material of that organism.
The whole human genome has now been studied and this will have great importance for medicine in the future.
The genetic material in the nucleus of a cell is composed of DNA.
DNA is a polymer made up of two strands forming a double helix.
A gene is a small section of DNA on a chromosome.
Each gene codes for a particular sequence of amino acids to make a specific protein.
Inheritance - Genes
Not all parts of the DNA code for proteins.
Non-coding parts switch genes on or off, so variations in these areas of DNA can affect how genes are expressed.
A change in the DNA structure may result in a change in the protein synthesised by the gene.
Mutations occur continuously. Most do not alter the protein or they alter it so slightly that the function is not affected.
A few mutations code for an altered protein with a different shape, affecting the function. This may be an advantage or a disadvantage.
Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.
The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as the phenotype.
If the two alleles are the same, the individual is homozygous for that trait, but if the alleles are different then they are heterozygous.
A dominant allele is always expressed in the phenotype even if only one copy is present.
A recessive allele is only expressed if two copies are present.
Most characteristics are the result of multiple genes interacting, rather than a single gene.
Direct proportion and ratios can be used to express the outcome of a genetic cross.
We can use Punnett Squares and family trees to help understand genetic inheritance.
Ordinary human body cells contain 23 pairs of chromosomes, 22 of these control general body characteristics only. But the sex chromosomes, the last pair, carry the genes that determine sex.
In human females the sex chromosomes are the same (XX).
In males the sex chromosomes are different (XY).
Some disorders are inherited.
Polydactyly is a dominant phenotype caused by a dominant allele which can be inherited from either or both parents.
It is a dominant genetic disorder resulting in extra fingers and toes.
Cystic Fibrosis is a recessive phenotype and is caused by recessive alleles which must be inherited from both parents.
Thick mucus that clogs lungs
Affects the quality of life
Reduces life expectancy
Screening for Genetic Disorders
Types of Screening
Carried out at around 15-16 weeks of pregnancy.
It involves taking some of the fluid from around the developing foetus.
The fluid contains foetal cells which can then be used for genetic screening
For + Against