National 5 Biology Unit 2 Cell Biology Section

National 5 Biology Unit 2 – Cell Biology Section 10 Variation and Inheritance

We will be learning… Describe how sexual reproduction maintains variation Give examples of discrete variation characteristics within a species (single gene inheritance) Give examples of continuous variation characteristics within a species (polygenic inheritance) Identify examples of dominant and recessive traits Explain what is meant by the term “genotype” and “phenotype” Use the terms “P”, “F 1” and F 2 Assign a genotype to an individual Explain what is meant by the terms “homozygous” and “heterozygous” Use Punnet squares to predict the inheritance of genes from parental generations through to the F 2 generation Understand the use of pedigree charts to investigate the inheritance of a characteristic Explain why expected phenotypic ratios are not always achieved

Species • Definition – • Group of organisms that are able to • interbreed • produce fertile offspring Differences between organisms of the same species is variation.

Variation in A Species List the features which differ within these dog breeds.

Types of Variation Some variation is continuous. • can have any one of a large range of values. • Wide range from one extreme to the other • e. g. height. This type of variation can be represented as a line graph or histogram

Types of Variation Some variation is discrete. • can only have one of a fixed number of options • there are distinct differences • e. g. eye colour This type of variation can be represented as a bar graph

Continuous and Discrete Variation Feature Eye colour Fill in the table to indicate whether the variation is continuous or discrete. Weight Shoe size Height Skin colour Tongue rolling Variation

Family resemblance Members of the same family often look similar. Which parent do these children look more like? If the son and daughter each have children of their own one day, will they also look like their parents? Why do members of the same family often look similar? Humans, like all organisms, inherit characteristics from their parents. How are characteristics passed on?

Genetic material When cells divide, it is essential that genes are copied into new cells. This ensures that new cell has a complete set of information. Genes are the basic unit of inheritance and are responsible for characteristics of an organism. Genes are located on chromosomes, each of which is made from a very long, tightly coiled molecule of DNA.

Alleles - Different versions of genes Chromosomes in a homologous pair contain the same type of genes that code for the same characteristics, such as eye colour. Each chromosome in the pair, however, may have a different version of the gene. allele for brown eyes For example, the version of a gene on one chromosome may code for brown eyes, whereas the version of the gene on the other chromosome may code for blue eyes. Each different version of a gene is called an allele for blue eyes allele.

Genotype and Phenotype • Definition – • Genotype – combination of alleles organism has for particular characteristic • - usually written as letters • Phenotype – the physical appearance of these characteristics

Homozygous alleles If the alleles for a characteristic in a homologous pair 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 If the alleles for a characteristic in a homologous pair 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 The characteristic shown in the individual who has heterozygous alleles will depend on which allele is dominant and which allele is recessive.

Dominant or Recessive? The phenotype for a particular characteristic depends on which allele is dominant and which allele is recessive. l Dominant alleles are always expressed in a cell’s phenotype. Only one copy of the dominant allele needs to be inherited in order for it to be expressed. Dominant alleles (e. g. brown eyes) are represented by an upper case letter (e. g. ‘B’). l Recessive alleles are only expressed in a cell’s phenotype if two copies of it are present. If only one copy is present, its effect is ‘masked’ by the dominant allele. Recessive alleles (e. g. blue eyes) are represented by a lower case letter (e. g. ‘b’).

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

Inheritance Terms

Homozygous cross

Heterozygous cross

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? l Pea plants grow quickly. l Pea plants are available in pure-breeding (homozygous) strains. l Many pea plant characteristics show discontinuous variation; they are either one form or another, with no intermediates. This means that their phenotypes are easily distinguishable.

Mendel’s Early Experiments

Monohybrid crosses The type of experiment that Mendel carried out, investigating just a single characteristic, is called a monohybrid cross. There are two alleles controlling pea shape. This means there are three possible genotypes that the F 2 generation of plants could inherit, leading to two possible phenotypes. Genotype Phenotype homozygous dominant SS smooth homozygous recessive ww wrinkly heterozygous Sw smooth The likelihood of a trait being produced during a monohybrid cross can be mapped out using a Punnett Square.

What are Punnett Squares?

Mendel’s laws of inheritance After his research, Mendel proposed two laws of inheritance. Mendel’s first law: the law of segregation l Alternate versions of genes (alleles) cause variation in inherited characteristics. l An organism inherits two alleles for each characteristic – one from each parent. l Dominant alleles will always mask recessive alleles. l The two alleles for each characteristic separate during gamete production. Mendel’s second law: the law of independent assortment l Genes for different characteristics are sorted independently during gamete production.

Finding the Genotype For some characteristics, the genotype of a homozygous recessive individual can be determined from their phenotype. For example, the allele for brown fur (B) in mice is dominant over the allele for white fur (w ). This means that all white mice must therefore have the genotype. But what about individuals that have brown fur? Is their genotype BB or Bb? A test cross can be used to determine whether an individual is homozygous or heterozygous for a dominant trait.

What is a test cross? During a test cross, an individual with an unknown genotype is crossed with a homozygous recessive individual. The phenotype of the offspring will reveal the unknown genotype. l If all the offspring display the dominant phenotype, then the parent of unknown genotype must be homozygous for the characteristic. l If half the offspring show the dominant phenotype, and half show the recessive phenotype, then the parent must be heterozygous for the characteristic.

Using Test Crosses to find Genotype

Family Tree Family trees can be used to show characteristics over generations Male This family tree shows inheritance of eye colour. Brown eyes is dominant to blue eyes. Female

Answers • Phenotype of person 2 – blue eyed • Phenotype of person 3 – brown eyed • Genotype of person 1 -Bb • Genotype of person 4 – Bb • Person 7 likely to be homozygous dominant because all the offspring show dominant characteristic Genotype of person 8 – bb Genotype of person 9 not certain because they have dominant phenotypes but both parents must be heterozygous as they have blue eyed child

Family Tree Example 2

Family Tree Example 2

Polygenic Inheritance Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin colour, eye colour and weight. • Genes involved in polygenic inheritance have an additive effect. This means that each dominant allele of each gene has an additive effect so therefore contributes to the characteristic controlled by the gene.

Glossary l allele – One version of a gene, found at a specific location along a chromosome. l dominant – An allele that is always expressed, even l l l 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. monohybrid cross – A cross in which one pair of characteristics is studied.

Glossary l phenotype – All the observable characteristics of an organism. l recessive – An allele that is only expressed if two versions of it are present in a cell. l variation – The difference between individuals within a population.

Anagrams

Multiple-choice quiz

Term Description F 1 The first generation of offspring of a particular cross F 2 The “grandchildren” of a particular pair of individuals F This stands for filial – literal translation means “son of a daughter” Homozygous The organism has two identical versions of the same allele. This organism is said to be true-breeding Heterozygous The organism has two different versions of the allele. This organism is not true breeding Polygenic A characteristic showing continuous variation which is controlled by the alleles of more than one gene

Term Description Allele A form of gene occurring at the same position on each of a pair of homologous chromosomes Phenotype The appearance or features of an organism, usually the effect of the genotype plus the effect of the environment Genotype The genetic make up of an organism. Different combinations of alleles produce different genotypes Pure(true) breeding A group of individuals which is homozygous for the gene studied. Dominant A form of a characteristic which masks another form. Often used to describe an allele Recessive A form of a characteristic which is masked by another (dominant) form. Often used to describe an allele P Generation The parents in a particular cross

Monohybrid Cross Practice • We inherit information from both parents and so we have two copies of genes for each characteristic (1 from each parent) • So we can have 3 combinations for the tongue rolling characteristic • TT – this is called homozygous dominant • Tt – this is called heterozygous • tt – this is called homozygous recessive This is called the organism’s genotype

Monohybrid crosses • When an organism is either TT or tt, it is said to be truebreeding • Most experimental crosses have true breeding parents which show different phenotypes of the same characteristic i. e. Genotype Phenotype TT can roll tongue tt cannot roll tongue • Crosses are always presented in the same way, but you must understand gamete formation first

When fertilisation occurs –the following results Sperm T Egg t T t Zygote

Chromosomes • When a sperm and an egg meet at fertilisation, the resulting zygote has 2 sets of chromosomes • this is because the sperm and the egg have one set of chromosomes each • the number of chromosomes must be 46 in body cells so that there is a complete set of chromosomes in every cell

Characteristic – ability to roll tongue letters used – T = can (dominant) t = cannot (recessive) A monohybrid cross • Parents • Phenotype TT x tt cannot • Gametes T or T t or t F 1 Tt Tt All have genotype Tt and the phenotype is they can roll their tongues

Examples • Mouse coat colour is determined by a gene • The black form of the gene is dominant and the white form is recessive • Use appropriate letters to show the offspring produced by a homozygous dominant and a homozygous recessive mouse

B – black (dominant) b – white (recessive) • Parents • Phenotype • Gametes BB x black B or B • F 1 all Bb • Genotype = Bb • Phenotype = all black bb white b or b

Example • Not all crosses are as straight forward as this • What offspring would you expect if you crossed a heterozygous mouse with a true breeding white mouse? • Use the same letters to construct a cross

B – black (dominant) b – white (recessive) • Parents • Phenotype • Gametes • F 1 Bb x black B or b ? ? ? ? bb white b or b

Example • When this situation arises, you cannot just look at the gametes and decide • Draw a cross like this: One set of gametes b b B Bb Bb b bb bb You have 2 Bb (black) and 2 bb (white) The ratio is 1: 1

Examples • (1) Red flowers are dominant (R) and white flowers are recessive (r). What offspring will you get if you cross a white flower with a homozygous red flower? • (2) Tall pea plants are dominant (T) and dwarf are recessive (t). What offspring will you get if you cross two heterozygous parents? • (3) Brown hair is dominant (B) to red hair (b). What offspring will you get if you cross a recessive (homozygous) with a heterozygous?

(1) • Parents • Phenotype • Gametes • F 1 rr x white r or r RR red R or R all Rr and red The capital letter always goes first in the genotype

(2) • Parents • Phenotype • Gametes • F 1 One set of gametes Tt x tall T or t Tt tall T or t One set of gametes One TT (tall), two Tt (tall) and one tt T t (dwarf) T TT Tt tt Phenotype ratio is 3: 1 Genotype ratio is 1: 2: 1

(3) • Parents • Phenotype • Gametes bb x red b or b Bb brown B or b • F 1 One set of gametes Two Bb (brown) and two bb (red) One set of gametes b b B Bb Bb b bb bb Phenotype and genotype ratio is 1: 1

The F 2 Generation • We have looked at the F 1 generation from a monohybrid cross, but what about the F 2? • This is the generation which is formed when at least one of the parents is from the F 1 generation

B – black (dominant) b – white (recessive) • • • Parents (P) phenotype gametes F 1 F 2 gametes BB x black B or B all Bb Bb x B or b bb white b or b Bb B or b • need to use the punnet square (grid)

F 2 Generation One set of gametes B b B BB Bb bb The phenotype ratio is 3: 1 (black: white) The genotype ratio is 1: 2: 1 (BB: Bb: bb)

Examples • Black coat (B) is dominant to white coat (b) in mice. What is the genotypic ratio and phenotypic ratio of the offspring produced in the F 2 generation from a dominant black mouse and a white mouse if one of the F 1 generation is crossed with another?

B – black (dominant) b – white (recessive) • • • Parents (P) phenotype gametes F 1 BB x black B or B bb white b or b One set of gametes b b B Bb Bb

B – black (dominant) b – white (recessive) • • F 1 generation crossed F 1 Bb x phenotype black gametes B or b • F 2 Bb black B or b One set of gametes B b B BB Bb b bb bb Genotypic ratio is 1: 2: 1 (1 x BB, 2 x Bb, 1 x bb) phenotypic ratio is 3: 1 (3 x black, 1 x white)

Back Cross (test crosses) • A homozygous dominant and a heterozygous mouse will look the same • this is because they both have the dominant gene (BB and Bb) • to find out what the genotype of an unknown mouse, you need to carry out a test cross • the mouse is crossed with a recessive mouse

Back Cross • Parents (P) • gametes • F 1 BB x bb OR Bb x bb B or B b or b B or b b or b B B b Bb Bb OR B b b Bb bb

Genetic information • Genes are part of chromosomes • every characteristic is controlled by 2 forms of a gene (one from mum and one form dad) • the forms of a gene are called alleles • each gamete carries one allele for a characteristic

Inheritance patterns • You do not always get the expected results • this is because fertilisation is a random process • e. g. two heterozygous mice should produce 3 black and 1 white offspring. They might produce 4 black mice