Inheritance & Genetics Flashcards
What is genetics?
Genetics is a branch of biology that studies inheritance and variation in organisms. Hereditary - Passing on of traits from one generation to the next. Genetic variation - The differences in alleles of genes found within individuals of a population
The Father of Genetics:
Gregor Mendel an Austrian monk, he was responsible the first major breakthrough in the study of hereditary by investigating the transfer of characteristics one generation to the next. Mendel’s experiments:
- There were a large number of pure-breeding pea plants available to Mendel, he raised and tested over 28 000 of them. He analysed the plant characteristics such as plant height, pod shape, pod colour, flower position, seed colour, seed shape and flower colour.
He was trying to establish what happens to the alternative forms of traits. Mendel suggested there are pairs of factors in each cell that control a characteristic such as colour.
Concepts of Inheritance
Inheritance results in both similarities and differences between parents and their offspring.
Chromosomes/Chromatin Network
Chromosomes are long, intertwined threads made up of a strand of DNA wound around proteins called histones. These are only visible in the nucleus when a cell is dividing.
Chromatin is a network made up of intertwined chromosomes. It is visible in the nucleus of a cell that is not dividing.
Genes:
Genes are passed from parent to offspring. A gene can be defined as a section of DNA (series of nucleotides) that controls a hereditary characteristic, ie it is the basic unit of hereditary in living organisms.
- Each chromosome has between several hundred and thousand genes.
- Nearly all somatic cells have an exact copy of all the genes in the organism. Mature red blood cells have nu nuclei; therefore no chromosomes and no genes.
- Because there are two of each kind of chromosomes, each cell contains two of each kind of gene. These two versions of a gene are known as alleles.
What is a genome?
A genome is the complete set of genetic instructions (genes) necessary to create an organism. The human genome project was started in the 1990s to find the position and structure of every gene in human DNA.
- Nearly every somatic cell in the body has a complete copy of a genome. Each species has a unique genome altough the genome of individuals of the same species may vary due to mutations.
- Determining the order in which bases occur in a DNA molecule is called DNA sequencing. Comparing DNA sequences shows how closely related different species are to each other.
How active are genes?
- Each cell contains a full complement of DNA, only genes that are needed are activated and the others are suppressed. Therefore, different genes are activated in different cells, creating the specific proteins that give a particular cell type its character.
- Some genes play a role in early development of the embryo and are inactive thereafter.
- Some genes are inactive in many types of cells, making proteins needed for basic functions. These are called housekeeping genes. Other genes, are inactive most of the time. Research is being done as to what switches genes on and off.
- A high proportion of genes are non-coding genes, ie they do not code for proteins and occur in-between the coding sections. Only 2% of our DNA codes for proteins.
Alleles
Alleles are variations of a gene. They control different versions of a characteristics found in the same locus (position) on homologous chromosomes. Different alleles carry different information about the same characteristic, eg for the gene determining coat color in cattle.
In any somatic cell there are only two of the possible versions of alleles for each gene - one inherited from the mother and one from the father.
How are alleles passed from parent to offspring?
Alleles are passed on from parents to offspring by way of chromosomes in the gametes that are made by meiosis in the sex organs. During meiosis the homologous chromosomes separate so each gamete receives only one allele of a pair for each characteristics.
Mendel’s Law of Segregation:
The characteristics of an organism are controlled by pairs of alleles which separate into different gametes as a result of meiosis. Therefore, only one allele from each gene pair is present in a gamete. Normal body cells (somatic cells) are diploid as they have:
- A pair of homologous chromosomes
- Two alleles for each gene which may be the same of different. After replication they have four alleles for each gene.
Sex cells (gametes) are haploid (n) as they have:
- One of a pair of homologous chromosomes
- One of each pair of alleles
What happens to the gametes?
During fertilization a male gamete, with its alleles, fuses with female and its alleles to form a diploid zygote with two sets of alleles.
The zygote divides and divides by mitosis to form an entire new organism, each cell with the same set of chromosomes and alleles as in the zygote.
Genotypes and Phenotypes:
A genotype is the genetic makeup of an organism.
A phenotype is the phsyical appearance of an organism, such as tallness. It is partly programmed by genes, ie its genotype, but also shaped by external factors such as exercise, diet and environment.
Dominant and Recessive Alleles
When two organisms are crossed with a pair of alleles that are different, one allele may be:
- Dominant, this characteristic is expressed in the phenotype of the offspring
- Recessive, this characteristic is suppressed in the presence of the dominant allele and not expressed in the phenotype of the offspring. A recessive allele will only be expressed in the phenotype of the offspring when no dominant allele is present.
Mendel’s Law of Dominance
In a cross of parents that are pure for contrasting traits, only the dominant trait will appear in the phenotype. Recessive alleles will always be masked by dominant alleles.
Homozygous and Heterozygous
After fertilization, the zygote and all the cells that develop from it will have two alleles for each gene, one from each parent. These will be in the same locus on each chromosome of the homologous pair.
- If the pair of alleles at a locus are the same, the organism is homozygous (purebred) fir that particular characteristics, e.g. both alleles will determine a black coat in cattle.
- If the pair of alleles at a locus are different, the organism is heterozygous (hybrid) for that particular characteristic, e.g. one allele determining a black coat and another determining a white coat in cattle. An individual that is heterozygous for a characteristic is called a hybrid.
