Variation and Inheritance 4 Variation Members of a

Variation and Inheritance (4)

Variation • Members of a species can interbreed and produce fertile young. • Individuals in a species are not all identical, but show slight differences. The differences that occur within a population (a group of organisms of the same species) are called variations.

• These genetic variations occur because the fusion of gametes, from separate parents, which take place during fertilisation is a random process.

• What different characteristics can you think of between us? – – – hair colour eye colour ear lobes tongue roller Height / weight / hand span / foot length

Continuous Variation • Some characteristics show continuous variation and they vary in a continuous way from one extreme to another. • The majority of a sample will be average for that characteristic and lie somewhere in the middle of the normal range of values.

Continuous Variation • Continuous variation is the result of POLYGENIC inheritance – more than one gene is responsible for the variation. • There is a range of values between a minimum and a maximum

– E. g. • • height weight hand span leaf length • Continuous variation can be shown by a bell-shaped curve on a line graph or histogram.

Discrete Variation • Some characteristics show discrete variation as the members of a species can be divided into groups based on this characteristic.

Discrete Variation • Discrete variation is usually the result of single gene inheritance – E. g. • • ear lobes (attached or unattached) tongue roller eye colour blood group

• Discrete variation can be shown by a bar chart or pie chart.

• All characteristics of living things are determined by our genetic information. The information is carried on the chromosomes found in the nucleus of every cell.

• Offspring receive half their genetic information from each parent. The information is carried in the form of a gene.

• Different forms of a gene are called alleles. E. g. the gene for eye colour has different forms: brown, blue, green, grey etc.

• Each parent pass on one allele for each characteristic to an individual. The alleles an organism contains is called its genotype. The alleles passed on may be the same or different 1 allele 2 alleles 1 allele

• An organism that inherits two identical alleles from each parent is said to be homozygous (true breeding). Blue Eyes

• An organism that inherits two different alleles from each parent is said to be heterozygous. Blue Eyes Brown Eyes

• The two alleles determine what the organisms looks like for a particular characteristic. This is called the phenotype. E. g. Eye colour. Blue Eyes Brown Eyes

• Many of the features possessed by an individual in terms of their phenotype have been caused by the interaction of the alleles of several genes. This is referred to as polygenic inheritance. • Most features of an individual phenotype are polygenic and show continuous variation. • Skin colour in humans and seed mass in plants are good examples of this.

• One allele always masks the effect of the other. This allele is called the dominant allele. The allele that is hidden is called the recessive allele. The organism will have the appearance (phenotype) of the dominant characteristic.

• Alleles can be represented using symbols. – A capital letter is used to represent the dominant allele (B) – A lower case letter is used to represent the recessive allele (b)

• In peas round seed is dominant to wrinkled seed. This can be represented in symbols. – Round – Wrinkled = dominant allele = recessive allele =R =r • There are three possible genotypes for seed shape. Plants could have: – 2 round alleles RR – One round and one wrinkled allele Rr – 2 wrinkled alleles rr

Shows the 2 alleles present What the organism looks like Possible genotype Phenotype RR Round Rr Round rr Wrinkled

• This is a cross between two parents with different phenotypes. Parents in a monohybrid cross are chosen because they are homozygous. It is usually represented in the form of a diagram. Symbols are used to represent some parts of the cross.

• P = Parents • F 1 = First generation of offspring • F 2 = Second generation of offspring produced by crossing the F 1 together. • X = Crossed together

• In plants red flowers are dominant to white flowers. • True breeding plants with red flowers were crossed with true breeding plants with white flowers. Follow the cross through to the F 2 generation.

• Parents (P): Red x White • Genotype: RR x rr • Gametes: all R all r

• F 1 Genotype: Rr • F 1 Phenotype: All red

• F 1 self crossed: Red x • Genotype: Rr • Gametes: R or r x Red Rr R or r

• F 2 Genotype: gametes R r R RR Rr r Rr rr • F 2 genotype ratio: 1 RR: 2 Rr: 1 rr • F 2 phenotype ratio: 3 red : 1 white

• In cattle black coat colour is dominant to red coat colour. • True breeding cows with black coats were crossed with true breeding cows with red coats. Follow the cross through to the F 2 generation.

• Parents (P): Black • Genotype: BB • Gametes: all B x x Red bb all b

• F 1 Genotype: Bb • F 1 Phenotype: All black

• F 1 self crossed: Black x Black • Genotype: • Gametes: Bb B or b x Bb B or b

• F 2 Genotype: gametes B b B BB Bb b Bb bb • F 2 genotype ratio: 1 BB: 2 Bb: 1 bb • F 2 phenotype ratio: 3 black : 1 red

Problems 1. Spotted Coat (S) is dominant to plain coat (s). Homozygous spotted dogs are bred with homozygous plain dogs. Follow the cross through to the F 2 generation.

Problems 2. Curly hair (H) is dominant to straight hair (h). A man homozygous for curly hair has a child with a woman with straight hair. Follow the cross through to the F 2 generation.

Problems 3. Pink flowers (P) are dominant to yellow flowers (p). Homozygous pink flowers are bred with homozygous yellow flowers. Follow the cross through to the F 2 generation.

Problems 4. Right handedness is dominant to left handedness. A man who is homozygous for left handed has a child with a woman who is left handed. Follow the cross through to the F 2 generation.

Sometimes the actual numbers obtained are different from the expected numbers. This is because: • fertilisation is a random process. • the sample was not big enough.

Genetic Counselling • Individuals who are heterozygous for a particular medical condition are said to be carriers. This means that while they are not affected, if the trait is recessive they may pass the allele onto their offspring. If the allele from their partner is also recessive they may have a child who is a sufferer. Known carriers are therefore often offered genetic counselling.

• Genetic counselling is the process by which patients or relatives, at risk of an inherited disorder, are advised of the consequences and nature of the disorder, the probability of developing or transmitting it, and the options open to them in management and family planning.