SECTION 5 - INHERITANCE National 4 & 5 – Multicellular Organisms

Why are we so similar yet different? We all belong to the same species We are similar in many ways But, we show a great deal of variation - continuous (e.g. height, weight) - discrete (e.g. eye colour, blood type) These similarities and differences are mainly determined by our genes

Learning Outcomes By the end of this section I will be able to - identify how genes determine characteristics - map out patterns of inheritance, including family trees - identify phenotypes and genotypes using punnet squares - define dominant and recessive characteristics, and identify homozygous and heterozygous individuals

Inherited Characteristics Our characteristics are determined by genetic information E.g. hair colour, eye colour, tongue- rolling Each parent passes on 1 piece of information for a certain characteristic The pieces of information from each parent may be the same or different A family tree can show how characteristics pass on through several generations

Phenotype An organisms appearance resulting from genetic information received from parents E.g - wing shape – wild-type, weak, strong - flower colour – red, white, purple - eye colour – green, blue, brown

Phenotype and genotype The overall appearance of an organism depends on two things: The full set of genes of an organism is called its genotype. All the observable characteristics of an organism are called its phenotype. 1.its genes (inherited characteristics) 2.the effects of the environment in which it lives. An organism’s phenotype therefore depends on its genotype plus environmental effects. phenotype = genotype + environmental effects

Genes & Alleles Each cell has two sets of chromosomes One set from each parent Each chromosome is made up of units called genes Each gene contains information for a particular characteristic Each gene normally has at least 2 different forms e.g. flower colour could be purple or white These different forms are called alleles

Inheritance studies Show the inheritance patterns of certain characteristics Usually done with an easily-bred species e.g pea plants Specific characteristics chosen for study e.g. flower colour When only 1 characteristic is examined, it is said to be a monohybrid cross Parents (P) are bred to produce the first filial generation of offspring (F 1 ) The F 1 generation are then interbred to produce the second filial generation (F 2 ) Phenotypes are observed to show how characteristics are passed on

Dominant/Recessive F 1 generation often shows only one characteristic coming through This characteristic is said to be dominant E.g. purple flower The other characteristic is often hidden It is said to be recessive e.g white flower BUT, in the F 2 both characteristics often appear Why?

Homozygous alleles If the alleles for a characteristic are the same, the organism is said to be homozygous for that characteristic. What colour eyes will these homozygous pairs of alleles produce? allele for brown eyes allele for blue eyes

Heterozygous alleles The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive. If the alleles for a characteristic are different, the organism is said to be heterozygous for that characteristic. What colour eyes will this heterozygous pair of alleles produce? allele for brown eyes allele for blue eyes ?

What eye colour? The allele for brown eyes is dominant over the allele for blue eyes. The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes. allele for brown eyes allele for blue eyes So, what colour will the eyes be of an individual who has both alleles for eye colour?

Inheritance terms

B is the gene for brown eyes b is the gene for blue eyes

Parents BBbb Body cell in father with a pair of genes for brown eyes Body cell in mother with a pair of genes for blue eyes

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb Body cell in father with a pair of genes for brown eyes Body cell in mother with a pair of genes for blue eyes Gametes

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes Gametes BB each sperm has a gene for brown eyes

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes Gametes BB each sperm has a gene for brown eyes bb each egg has a gene for blue eyes

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes Gametes BB each sperm has a gene for brown eyes bb each egg has a gene for blue eyes At fertilization There are 4 possible combinations of sperm and egg

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes Gametes BB each sperm has a gene for brown eyes bb each egg has a gene for blue eyes At fertilization There are 4 possible combinations of sperm and egg BBBB

B is the gene for brown eyes b is the gene for blue eyes Parents BBbb body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes Gametes BB each sperm has a gene for brown eyes bb each egg has a gene for blue eyes At fertilization There are 4 possible combinations of sperm and egg Bb BBBB b All the children of this F1 generation have genotype Bb and phenotype brown eyes

Parents (F1) father with brown eyes mother with brown eyes Gametes At fertilization

Parents (F1) Bb father with brown eyes mother with brown eyes Gametes At fertilization

Parents (F1) Bb father with brown eyes mother with brown eyes Gametes Bb Bb At fertilization

Parents (F1) Bb father with brown eyes mother with brown eyes Gametes Bb Bb At fertilization BbBb B b

Parents (F1) Bb father with brown eyes mother with brown eyes Gametes Bb Bb At fertilization BBBb bb BbBb B b

Parents (F1) Bb father with brown eyes mother with brown eyes Gametes Bb Bb At fertilization BBBb bb BbBb B b A child who inherits the genes BB will have brown eyes A child who inherits the genes Bb will have brown eyes A child who inherits the genes bb will have blue eyes

Monohybrid Cross In this example purple is dominant to white P = purple p = white Let’s assume that both parents are homozygous (true breeding) One is PP (purple), the other pp (white) The F 1 offspring would gain a purple allele (P) and a white allele (p) - therefore only purple flowers in the F 1 generation (Pp) When the F 1 interbreed they could put forward either a purple (P) allele - or a white (p) allele A punnet square allows us to map out the possible combinations in the F 2 We discover that the numbers produced are 3 purple:1white This is called the phenotypic ratio

Green flower (G) is dominant Yellow (g) is recessive All the F1 generation have the Gg genotype They are all therefore green Then the F1 plants are crossed The results of the F2 are: 1GG:2Gg:1gg - this is called the genotypic ratio - 3 green:1 yellow - this is the phenotypic ratio 1 GG = green 2 Gg = green 1 gg = yellow

Expected vs Observed ratio With a monohybrid cross, a 3:1 ratio would always be expected in the F 2 generation However, there is often a difference between the expected and the observed results This is because fertilisation is a random process, involving an element of chance A punnet square only shows the likely outcomes, not what will actually occur In real life, if you toss a coin 20 times, you would expect 10 tails:10 heads – rarely occurs GenerationGreenYellow Parents (P)6 (GG)6 (gg) F1F1 198 (Gg) F 1 crossGg x Gg F2F Using the previous example the 200 offspring from the F 2 generation doesn’t exactly match a 3:1 ratio

Using a test-cross To help identify it’s genotype, it is crossed with a white flower (a white flower can only have a pp genotype) In this example purple (P) is dominant to white (p) We have a flower that is purple, but don’t know if it is homozygous (PP) or heterozygous (Pp) The offspring produced should prove what the unknown genotype is On occasion, an organisms genotype may be uncertain, but needs to be identified. A test-cross is used to prove an unknown genotype

For example, when a red snapdragon plant is crossed with a white snapdragon plant, all the offspring flowers are pink. What is incomplete dominance? Sometimes two different alleles are neither fully dominant or recessive to each other. In heterozygous individuals, this creates a phenotype that is a mix of the other two. This is called incomplete dominance. - because both the red and white alleles are expressed.

What is co-dominance? The human blood group system is controlled by three alleles: A, B and o. In heterozygous individuals who have A and B alleles, both are fully expressed This is called co-dominance. A and B are dominant while o is recessive. - creating an extra phenotype (AB)

Co-dominance in humans

The life and work of Gregor Mendel

Mendel’s experiments Over seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful? Pea plants grow quickly. Pea plants are available in pure- breeding (homozygous) strains. Many pea plant characteristics show discrete variation; they are either one form or another. This means that their phenotypes are easily distinguishable.

What are sex chromosomes? Humans cells contain one pair of sex chromosomes, which control gender. Males have one X and one Y chromosome (XY). Y chromosomes are small and contain 78 genes X chromosomes are larger and contain 900– 1,200 genes. Because females can only produce X gametes, it is the sperm that determine the sex of the offspring. X chromosome Y chromosome Females have two X chromosomes (XX).

Boy or girl?

Multiple-choice quiz

Anagrams

Glossary (1/4) acquired – A characteristic of an organism that depends on environmental factors. allele – One version of a gene, found at a specific location along a chromosome. carrier – An individual with a recessive allele, whose effect is masked by a dominant allele. characteristic – A specific feature of an organism, such as eye colour. co-dominance – A situation where two alleles are equally dominant. continuous – Variation represented by a continuous range of values and which can be measured.

Glossary (2/4) discontinuous – Variation represented by discrete categories. dominant – An allele that is always expressed, even if the cell only contains one copy. gene – The unit of inheritance. genotype – The full set of genes of an organism. heterozygous – Having two different alleles of a specific gene. homologous chromosomes – A matched pair of chromosomes that carry genes for the same characteristics. homozygous – Having two identical alleles of a specific gene.

Glossary (3/4) incomplete dominance – A situation where two alleles are both partially expressed, producing an intermediate phenotype. inherited – A characteristic of an organism that depends on its genes. monohybrid cross – A cross in which one pair of characteristics is studied. phenotype – All the observable characteristics of an organism. recessive – An allele that is only expressed if two versions of it are present in a cell.

Glossary (4/4) test cross – A situation where an individual with an unknown genotype is bred with a homozygous recessive individual to reveal the unknown genotype. variation – The difference between individuals within a population.