The Inheritance of Genes: Patterns, Principles, and Genetic Diseases

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The inheritance of genes follows patterns- Gregor Mendel was an Austrian monk who discovered the principles of inheritancewith experiments in which a large number of pea plants were crossed. 1) First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics

2) Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics

3) These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed 

Mendel discovered the following things:

1) When he crossed two different purebred varieties together the results were E.G. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants

2) When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1. E.G. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1 

Mendel drew the following conclusions: 1)Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)

2) Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles).3) Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)4)Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm

5) For each factor, one version is

CERTAIN RULES Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene  Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene* Principle of Dominance:

HAPLOID GAMETES: --gametes are haploid so contain only one allele of each gene-- the two alleles of each gene separate into different haploid nuclei durinf meiosis--- Gametes are haploid sex cells formed by the process of meiosis males produce sperm and females produce ova. Types of Zygosity= Fusion of gametes results in diploid zygotes with two alleles of each gene that can be the same allele or different alleles or different alleles----Gametes are haploid, meaning they only possess one allele for each gene--When male and female gametes fuse during fertilisation, the resulting zygote will contain Exception: Males have only one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY) For any given gene, the combination of alleles can be categorised as follows:  maternal and paternal alleles are the same, homozygous for that gene 2) maternal and paternal alleles are different, heterozygous for that gene 3) Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene MODES OF INHERITANCE dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects -Genotype= (allele combination) for a specific trait  typically be either homozygous= BB or bb heterozygous= Bb  Phenotype= the physical expression ( 50%, 25%, 100%) black, white, blue - BB dominant and Bb recessive  CO-DOMINANCE is feathering in chickens – black (CB) and white (CW) feathers create a speckled coat (CBCW)Human red blood cells The letterI is used to represent the different antigenic forms (isoantigens) A allele = 

Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype Genetic diseases can be caused by recessive, dominant or co-dominant alleles.     

An autosomal recessive genetic disease will only occur if both alleles are faulty--Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers) --- An example of an autosomal recessive genetic disease is cystic fibrosis.     

An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder-- Homozygous dominant and heterozygous individuals will both develop the full range of disease symptomsAn example of an autosomal dominant genetic disease is Huntington’s disease.   

If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur-- However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal alleleAn example of a genetic disease that displays co-dominance is sickle cell anaemia.   

Cystic Fibrosis= 1)Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7 2) 3) This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts 4)

Huntington’s Disease= 1) Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4.

2) The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats). 

3)More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)

4)When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration

5)This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years) 

6)Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia

There are over 4,000 identified single gene defects that lead to genetic disease, but most are very rare

Any allele that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring

Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease.      Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele SEX LINKED GENES= refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y) The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes) 2) The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes) Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome

Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY) This leads to the expression of sex-linked traits being predominantly associated with a particularly gender- As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous 1) Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease) 2) Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits 3) X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele Trends always hold true for X-linked conditions: Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers 2) Males will Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele) Red-green colour blindness and haemophilia are both examples of X-linked recessive conditionsHaemophilia:  XH = unaffected (normal blood clotting) ; Xh = affected (haemophilia) Colour blindness:  XA = unaffected (normal vision) ; Xa = affected (colour blindness)  Haemophilia is a genetic disorder the body’s ability to control blood clotting is impaired --The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome---When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time--Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.G. Haemophilia A = factor VIII) Red-Green Colour Blindness  genetic disorder an individual fails to discriminate between red and green hues--caused by mutation to the red or green retinal photoreceptors, which are located on the X chromosomes

Gene mutation is a change to the base sequence of a gene that can affect the structure and function of the protein it encodes Mutations can be Examples of factors which can induce mutations include: Radiation – e.G. UV radiation from the sun, gamma radiation from radioisotopes, X-rays from medical equipment. ChemicalBiological Agents – e.G. Bacteria (such as Helicobacter pylori), viruses (such as human papilloma virus) --- Agents which increase the rate of genetic mutations are called Mutagens which lead to the formation of cancer are more specifically referred to as carcinogens.     A pedigree is a chart Males are represented as squares, while females are represented as circles 2) Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected  3) A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line  4)Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left) Autosomal Dominant = If both parents are affected and an offspring is unaffected, the trait must be dominant (parents are both heterozygous) All affected individuals must have at least one affected parent.  If both parents are unaffected, all offspring must be unaffected (homozygous recessive) Autosomal Recessive If both parents are unaffected and an offspring is affected, the trait must be recessive (parents are heterozygous carriers) 

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