Characteristics & Inheritance Unit 1.3 Year 10 Human Biology

Genetics Gregor Mendel was a monk who carried out experiments on pea plants. Although he did not know about DNA and genes, he was able to develop theories that have formed the basis of modern genetics.

wrinkled round Punnett square

Dominant / Recessive inheritance In our DNA there are section of nucleotides that code for our characteristics. These sections are called genes. Genes occur in pairs, and are inherited from our parents. There are different forms of these genes – and these are called alleles.

Dominant / Recessive inheritance The allele that is expressed or observed in an individual is the dominant allele. The characteristics that is hidden is the recessive allele. As genes occur in pairs an individual can have both a recessive and dominant allele.

Allele combinations Homozygous – having the same alleles for a gene. 2 dominant alleles 2 recessive alleles Heterozygous – having two different alleles for a gene 1 dominant and 1 recessive allele

Genetics rules To represent the dominant allele you use CAPITAL letters. ie: B, A, E, H, D, R, T Lowercase letters are used to represent the recessive allele. ie: b, a, e, h, d, r, t Putting the two together you can show the alleles for one characteristic that a person has. This is their GENOTYPE ie: Bb, Tt, AA, ee

Genetics rules Punnet squares are used to determine the probability of a characteristic being shown in the offspring (progeny) You can determine the genotype (the alleles) and then determine the PHENOTYPE (the expressed characteristic) Percentages & Ratios are used to show the probabilities of the genotypes and phenotypes Practise the Dominant & Recessive problems

Incomplete dominance There are some alleles for genes that do not follow the dominant / recessive pattern. These alleles show an incomplete dominance, meaning both alleles can be expressed in the phenotype of the individual. You can determine a THIRD phenotype from incompletely dominant alleles. This new phenotype usually a blending of two alleles

Incomplete Dominance rules As the two alleles are equally dominant, you must use two different CAPITAL letters ie: RW, Example: Red and White flowers are incompletely dominant in snapdragon plants. RR = Red WW = White RW= Pink Practise the Incomplete Dominance questions

Sex determination Humans have 23 pairs of chromosomes. 22 pairs of these chromosomes determine body characteristics and are termed SOMATIC chromosomes. 1 pair determine the sex of a human XX – Female XY - Male

Sex linkage There are a few genes that are located on the X and Y Chromosomes. These genes are inherited differently as they rely on the sex of an individual. Examples Red-green colour blindness Haemophillia Duchenne’s muscular dystrophy

Sex linkage rules As these alleles are found on the sex chromosomes you MUST show them on there! ie: XNXN = Normal female XNXn = Carrier female XnXn = Colour blind female XNY = Normal male XnY = Colour blind male Practise the Sex Linkage questions

Chromosome abnormalities A mutation is defined as a spontaneous change in the DNA of an individual They can occur within a chromosome, affecting one or many genes A chromosomal mutation occurs when there is a problem separating chromosomes during mitosis or meiosis These mutations can change the genotype and phenotype of an individual