Genetic Crosses & Sex Determination How is sex determined in offspring? What is a genetic diagram and how can it be used to investigate inheritance?

Sex determination Look at this karyotype. Can you tell what sex this individual is? One of the 23 pairs* of chromosomes are known as the sex chromosomes, called X and Y. This is the only example where the two chromosomes in the pair don’t have to carry the same genes

Sex determination – The X & Y Chromosome The X chromosome is visibly larger than the Y and carries more genes. They were named in order of their discovery – not because of their similarity to the letters. A female’s cells will carry 2 X chromosomes (XX), whilst a male’s will carry an X and a Y (XY)

Sex determination – The X & Y Chromosome X Chromosome Contains around 1500 genes. e.g. Rhodopsin – a light sensitive protein found in light receptor cells The genes on the X chromosome are not specifically used for the development of female traits Y Chromosome Contains around 350 genes. The most important of these is the SRY gene, which coordinates the development of the testis in males. The production of testosterone brings about development of male traits

What determines sex? Which process produces gametes? Meiosis How many copies of each chromosome will each new gamete contain? One When a female produces eggs, which type of sex chromosome can her gametes contain? X only When a male produces sperm, which types of sex chromosome can these gametes contain? X or Y What proportion of the sperm a male produces will contain X and Y chromosomes? ½ will contain X and ½ will contain Y On fertilisation, how many sex chromosomes will a zygote contain? Two Can you explain why there are roughly the same number of boys and girls born worldwide each year?

X X Y X Y X X Punnet diagrams We can use a genetic diagram (also called a punnet square) to calculate the probability that certain characteristics will appear in the offspring of 2 parents. In the example of sex determination, we first write out the possible gametes each parent can produce, i.e. which type of sex chromosome could they contain. Next we use the punnet square to show all the possible combinations that could occur due to random fertilisation Each time fertilisation occurs there is an equal probability of each possible combination What is the probability that offspring will be male or female? If these parents had 4 children, would they definitely get 2 boys and 2 girls? This shows us the proportion of each ‘type’ of gamete formed Clearly females can only produce eggs with X chromosomes in, so it is a bit silly (and misleading) writing it out twice

Punnet squares help us work out the possible gametes & zygotes X Y X All eggs produced will contain an X chromosome as all females are ‘XX’. Since there is no other possibility, only one column is needed On production of millions of sperm cells by meiosis, exactly half should contain an X chromosome and the other half will contain a Y X X Y There are only 2 possible outcomes, and this method shows the probability of each possibility very clearly 50%

Parents’ phenotype Male Female Parents’ genotype X Y X X Gametes’ genotype X Y X Offsprings’ genotypeX X X Y Offsprings’ phenotypesFemale Male Ratio of phenotypes 1 : 1 A genetic diagram shows the whole story

Genetic Crosses You have 2 copies of each gene in the human genome – 1 inherited from each parent Each gene may come in different forms, called alleles The combination of alleles you possess for a particular gene/characteristic is called your genotype The characteristic determined by this genotype is called the phenotype e.g. blue eyes

Eye colour There are different alleles of the gene for eye colour. When expressed, these would give you brown eyes, blue eyes or green eyes etc.

Genotype When describing genotype, letters are used to represent genes. For eye colour, we might use the letter ‘B’, and we can show different alleles by changing the case. The genotype of a person with 2 brown eye alleles would be BB, and their eyes would be…brown! The genotype of a person with 2 blue eye alleles would be bb, and their eyes would be…blue!

What if someone has one brown eye allele and once blue eye allele? Their genotype would be Bb, but because the B allele is ‘stronger’, the person would have brown eyes We say that the brown eye allele is dominant, and this is why it is shown by a capital letter - B. The blue eye allele is recessive, so is represented by a lower case letter – b.

More terminology… Special terms are used to describe an individual’s genotype for a particular characteristic or trait. Heterozygous – this means a person has two different alleles for a particular gene e.g. Bb Homozygous – this means that for a particular gene, a person has two copies of the same allele e.g. bb or BB. The word homozygous must always be followed by ‘recessive’ or ‘dominant’ in order to complete the description.

A dominant allele will always determine the phenotype, regardless of the other allele present i.e. the phenotype will be the same whether the person is homozygous for the dominant allele or heterozygous. A recessive allele will only determine phenotype if both copies of the allele are recessive i.e. the person is homozygous for the recessive allele. But what if two alleles are equally strong?

Codominance For some genes, neither allele is recessive, so the phenotype of someone who is heterozygous for that gene will be a mixture. ABO blood groups There are 2 codominant alleles – A & B An AB heterozygous individual will show characteristics from both alleles. They are not blood group A or B but AB

Codominance For some genes, neither allele is recessive, so the phenotype of someone who is heterozygous for that gene will be a mixture. Flower colour in Snapdragons There are 2 codominant alleles for the colour gene – R (red) and W (white) A heterozygous individual will show characteristics from both alleles. They will not be red or white, but pink!

Genetic diagrams describe patterns of monohybrid inheritance Monohybrid inheritance refers to characteristics that are controlled by just one gene. What would happen if a blue eyed male had a baby with a brown eyed female (let’s make her homozygous dominant) A genetic diagram allows us to predict the possible phenotypes of their offspring. Blue eyes - bbBrown eyes - BB ?

Using Genetic diagrams Parents’ phenotype Parents’ genotype Gametes’ genotype Offsprings’ genotype Offsprings’ phenotypes Ratio of phenotypes Blue eyes Brown eyes bbBB bB Bb Brown eyes 1 – all offspring will have brown eyes Be careful not to carry out the same cross twice or cross any of the gametes with another from the same parent! Each of these steps may be mark- worthy in an exam so practice & don’t forget any steps!

Using Genetic diagrams – another example Parents’ phenotype Parents’ genotype Gametes’ genotype Offsprings’ genotype Offsprings’ phenotypes Ratio of phenotypes Brown eyes Bb B b Bb BB Bb bb Brown eyes Brown eyes Brown eyes Blue eyes 3 (brown) : 1 (blue) Be careful not to carry out the same cross twice or cross any of the gametes with another from the same parent! Make it really clear which genotype belongs with which phenotype If giving a ratio or probability – make it clear which phenotype(s) you are referring to

Using Genetic diagrams Parents’ phenotype Parents’ genotype Gametes’ genotype Offsprings’ genotype Offsprings’ phenotypes Ratio of phenotypes Brown eyes Bb BB Bb bb Brown eyes Brown eyes Brown eyes Blue eyes 3 (brown) : 1 (blue) BB Bb bb Bb BbBb If you prefer, you can use a punnet square to work out the possible offspring genotypes

Brown eyes - BBBlue eyes - bb On your whiteboards, predict the possible genotypes & phenotypes of these 2 individuals B b BbBb Brown eyes heterozygous The probability of these parents having a brown eyed child is…1 You’ll soon realise there is no need to draw 4 boxes – why? Punnet squares really just show us the possible outcomes at fertilisation In the exam you will need to draw & interpret full genetic diagrams Gametes?

Brown eyes - BBBlue eyes - bb B b BbBb Brown eyes heterozygous If both parents are homozygous there will only ever be one possible genotype for their offspring. If one is HZD and the other is HZR they will always have heterozygous offspring. If both parents are homozygous for the same allele then clearly their children will be too – this is the basis of pure breeding – which isn’t necessarily a good thing! This method clearly shows that all the offspring of this couple will be heterozygous. The probability of their child being heterozygous is 1 or 100% Using Punnet squares like this ensures you always get the correct ratio/proportion of genotypes/phenotypes in the offspring. You’ll also learn the patterns a lot better too!

Brown eyes - BbBlue eyes – bb Homozygous recessive On your whiteboards, predict the possible genotypes & phenotypes of these 2 individuals B b b BbBb bb Brown eyes heterozygous Blue eyes Homozygous recessive What if the mother was heterozygous? Gametes? The probability of these parents having a brown eyed child is…0.5 or 50% The probability of them having a blue eyed child is…0.5 or 50% In other examples you may be given the genotypes or phenotypes of the offspring and be expected to work out the genotype of the parents!

Now try these!

GG g Gg grey white G g tt small grey small t t R r R Red white R R R r r r r Red White When you cross heterozygotes, you always get a 3:1 phenotypic ratio

What about codominance? Blood type: 2 codominant alleles – A & B 1 recessive allele – O Blood typeGenotype AAA or AO BBB or BO AB OO What is the probability that: 1.2 AB parents will produce AB offspring? 2.2 AB parents will produce BB offspring? 3.An AO and a BO parent will produce an O type offspring? 4.A type O parent and a parent with the genotype AO will produce type A offspring?

Family Pedigrees A pedigree is a diagram that shows how a genetic condition has been passed through several generations of a family They can be used to predict the probability of a condition being passed to future generations and they’re also very interesting!

Family Pedigrees Cystic Fibrosis Caused by a faulty allele normally involved in mucus production Sufferers produce large amounts of very sticky mucus which affects the lungs, the digestive system & the reproductive system Treated with antibiotics, enzymes and daily physiotherapy. Caused by a recessive allele. Heterozygous individuals have a working copy of the gene so don’t show symptoms but can still pass the faulty allele to their children – they are known as carriers. There is no cure, but gene therapy may provide a solution in the future.

Family Pedigrees Cystic Fibrosis Affected male Affected female Unaffected male Unaffected female Horizontal lines with no joints show parents Vertical lines connect to biological offspring of these parents You always get a key!

Family Pedigrees Cystic Fibrosis Affected male Affected female Unaffected male Unaffected female How many generations are shown in this pedigree? 2.What evidence in the pedigree suggests that cystic fibrosis is determined by a recessive allele? 3.What are the genotypes of individuals 3, 4, and 11? Explain your answers 4.Draw genetic diagrams to work out the probability that the next child born to individuals 10 & 11 will i) be male, ii) suffer from cystic fibrosis

Family Pedigrees Cystic Fibrosis Affected male Affected female Unaffected male Unaffected female If a child (e.g. 8) has a different phenotype to both parents then the disease causing allele must be recessive. 3.3 & 4 must be heterozygous (Cc) as they are not sufferers but have a child with cystic fibrosis. 11 must be homozygous recessive (cc) as they are a sufferer. 4. i) P (be male) = 0.5 or 50%, ii) P (CF) = 0.5

Family Pedigrees Cystic Fibrosis Affected male Affected female Unaffected male Unaffected female Parents’ phenotype Parents’ genotype Gametes’ genotype Offsprings’ genotype Offsprings’ phenotypes Ratio of phenotypes Unaffected male Affected female C cc Cc c C cc Unaffected Affected 1 : 1

Family Pedigrees Huntington’s disease A disease of the nervous system Cause muscle spasms & death Symptoms don't appear until middle age, by which time a sufferer may already have had children Incurable Caused by a dominant allele

Pedigree for Huntington’s disease Because Huntington’s is caused by a dominant allele we can make some assumptions All sufferers must possess at least one dominant allele If someone was homozygous dominant, all their offspring will be sufferers If a sufferer has non- suffering siblings, their parent sufferer must have been heterozygous Even if only one parent is a sufferer, there is at least a 50% chance that any offspring will inherit the disease

Do ‘Genetics Questions’ sheet. Questions – Page 206 – 207 Exam Questions