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You receive your genetic instructions from each of your parents. The double set of genetic instructions, one set from the mother and one set from the father, affect you throughout your life, long after your parents are dead. You will yourself transmit a set of genetic instructions to your children. All this involves gametes. The genetic instructions from your mother was in the nucleus of her egg cell and the set from your father was in the nucleus of a sperm cell. Egg and sperm cells are gametes. Gametes are called germline cells.
During fertilisation, the nucleus of the sperm cell fuses with the nucleus of the egg cell to form the nucleus of the fertilised egg which contains the double set of instructions for a unique individual. The only exception to this are identical twins which result from cells derived from a single fertilised egg and so share the same genetic instructions. Gametes are the only cells that can cross the generation gap. All the other cells that make up a multicellular organism are known as somatic cells and they die when the organism dies. The link across generations is through the deoxyribonucleic acid (DNA), the material of the genetic instructions or genes, and these are transmitted between generations by gametes. Genetic instructions within your cells include some that have been passed down from your great-great-grandparents and earlier generations, who may have been born in another part of the world. The double set of genetic instructions present in an organism makes up its genotype. A particular genotype is generated when an egg is fertilised by a sperm. The visible expression of the genotype in the physical, biochemical and physiological characteristics of a person is called the phenotype. Other examples of phenotypes are traits such as blood group A, ability to make pigment and colour blindness. |
Genetic instructions are present in the DNA in the nucleus of each somatic cell. Each different instruction is a specific gene made of DNA. Genes are organised into larger structures known as chromosomes with each chromosome carrying a large number of genes.
How many? Each species has a characteristic number of chromosomes in its somatic cells (humans, 46). This number is often denoted as ‘2n’ and is referred to as the diploid number. Gametes contain half the diploid number, the haploid number (n). Human chromosomes A complete set of chromosomes can be organised into a karyotype. The 46 human chromosomes from a normal human male can be arranged into 23 pairs of chromosomes, consisting of 22 matched pairs and one ‘odd’ pair that is made up of one larger X chromosome and a smaller Y chromosome. In a normal female, a similar arrangement is seen, except that there are two X chromosomes and no Y chromosome. The pair of chromosomes that differs between the sexes makes up the sex chromosomes. Not all animal groups have this system. The 22 matched pairs of chromosomes present in both males and females are termed autosomes. These different autosomes can be distinguished by:
The members of each matching pair of chromosomes (e.g. two number-5 chromosomes) are said to be homologous. Non-matching chromosomes (e.g. number-5 chromosome and number-14 chromosome) are said to be non-homologous. Each chromosome has a constriction that is known as a centromere. The centromere forms the attachment point for the spindle fibres that are necessary for the orderly movement of chromosomes during both cell division (mitosis) and gamete formation (meiosis). This orderly movement of chromosomes ensures that daughter cells formed by mitosis will each have a double (diploid) set of chromosomes and that gametes formed by meiosis will contain a single (haploid) set of chromosomes. Analysing karyotypes Mistakes in chromosome numbers or abnormalities of single chromosomes can produce congenital disorders. Specific chromosome abnormalities are associated with various cancers and these chromosome changes can indicate the likelihood of remission. Computers can be used to analyse chromosomes and to automatically generate karyotypes to identify various conditions. |
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Heredity is the study of inheritance. The principles of heredity and patterns of inheritance were established by an Austrian monk, Gregor Mendel (1822–84), in the 19th century. Classical genetics deals with studying the mechanism and patterns of inheritance through the transmission of coded chemical instructions from one generation to the next. Early studies focused on individual genes – segments of deoxyribonucleic acid (DNA) – that directed the formation of particular structural and functional proteins of cells. Increasingly, however, research has shown that genes may code for more than one kind of protein and that they interact in their expression – that is, in what they do. The sum of all the DNA in the cell of an organism is its genome. Genomes differ between species although there may be similarities. The study of genomes of organisms is termed genomics.
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Complete Dominance
The height of the pea plants in Mendel's experiments are an example of complete dominance, where the dominant allele for tallness masks the recessive allele for shortness. To decide whether a trait is dominant or recessive, the phenotype of a heterozygous organism is identified. The trait that is expressed in this phenotype is the dominant trait, The alleles that control dominant traits are usually symbolised by a capital letter; for example, the allele F controls the dominant free ear lobes trait in humans. Alleles that control recessive traits are symbolised by the lower case of the same letter; for example the allele f controls the recessive attached ear lobes in humans. It is important to note that in genetics organisms can be carriers for certain traits. This organism would have to be a heterozygote, that does not express the trait in their phenotype. Some of the recessive traits are harmless e.g. straight hair line, blood type O, others are more significant e.g. carriers for the cystic fibrosis or albinism allele. |
Co-Dominance
The ABO gene, located on the number-9 chromosome, has three alleles that determine antigen production, Depending on which antigens are present, blood is typed as group A, B, AB or O. A person who has the blood group AB is said to be heterozygous and because both traits are expressed in the heterozygote, these two alleles show co-dominance. Alleles showing co-dominance are denoted with a capital letter with a super script added to it to distinguish between them. Shorthorn cattle are another example of this. Where both red coat colour and white coat colour are co-dominant. This means that both colours are expressed in the form of a roan coloured coat which is a mixture of read and white hair. |
Genetic crosses, Lethal Genes & Polygenics | |
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Environmental conditions can affect gene expression. If you look at a set of identical twins they may have slightly different weights and heights despite having the same genotype. As well as the environment, scientists are discovering more and more about chemical modifications to DNA by means of DNA methylation. The epigenome is what refers to the DNA plus the chemical compounds such as methyl groups that are attached to it and influence gene expression. The study of this is known as epigenetics. There are many examples of this which are explained further in the PowerPoint below.
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Epigenetics and Environmental Influences on Phenotypes | |
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Sometimes single characteristics are controlled by the alleles of two or more genes interacting with one another. A characteristic controlled by more than one gene is known as a polygenic characteristic, and its transmission is known as polygenic inheritance. Typically, the phenotypes produced by polygenes form many classes that show continuous variation. This means that if a large sample is taken, the values form a continuum. In humans good examples of this are height, eye colour and skin colour. Other examples are the fat content of cows milk. the mass of bean seeds and maximum speed of thoroughbred horses. In comparison traits that are controlled by a single gene are said to be monogenic and typically show discontinuous variation; that is, the expression of the single gene involved produces just a few discrete and non-overlapping phenotypes, often just two categories,
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Dihybrid crosses are where there's two genes in action! In a dihybrid cross between two parents purebred for two independently assorting characteristics, the F1 offspring are heterozygous for both traits and show the combination of dominant phenotypes. In a cross between F1 individuals, the F2 offspring are predicted to shoe four combinations of phenotypes, dominant-dominant: dominant-recessive : recessive-dominant : recessive-recessive in the ratio of 9:3:3:1. This type of inheritance highlights Mendel's rule of independent assortment, where the inheritance pattern of one trait will not affect the other.
A dihybrid test cross may be used to identify whether or not two genes are linked. The closer two genes are, the less chance there is that crossing over will separate the parental arrangement of alleles. However, when the genes are linked, parental gametes are formed in addition to smaller numbers of recombinant gametes.
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Genetic screening is used to identify specific (often harmful) genes in embryos, children, and adults. It raises many ethical issues, especially for embryo testing.
The genetic screening of gametes, embryos, children, and adults for some diseases is now possible. Genetic screening has many applications including the detection and treatment of diseases. Whilst genetic screening has many positive applications, it raises a number of ethical issues. This is particularly the case for the screening of embryos and fetuses because it may result in the destruction of embryos and fetuses is that have genetic defects, or even an undesirable genotype ( e.g. wrong sex). In Australia genetic screening is carried out on newborn babies with consent from the parents. Over 99% of parents do consent to this and it checks for inherited genetic disorders that do not show any symptoms at birth, such as PKU, cystic fibrosis, congenital hypothyroidism etc. |
A pedigree can be made to show the inheritance of a particular trait. Certain symbols are used. The pattern of inheritance of a trait in a pedigree may provide information about the trait. The pattern may indicate whether:
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Autosomal dominant pattern
An idealised pattern of inheritance of an autosomal dominant trait includes the following features:
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Autosomal recessive pattern
Albinism (a) is an autosomal recessive trait. An idealised pattern of inheritance of an autosomal recessive trait includes the following features:
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X-linked dominant pattern
Rickets is an X-linked dominant trait, controlled by the HYP gene located on the X chromosome and is expressed even if a person has just one copy of the allele responsible. An idealised pattern of inheritance of an X-linked dominant trait includes the following features:
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X-linked recessive pattern
An idealised pattern of inheritance of an X-linked recessive trait includes the following features:
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