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KS4 Biology Inheritance 1 of 36 © Boardworks Ltd 2004 Contents Inheritance All about alleles Homozygous cross Heterozygous cross Using a test cross Co-dominance 2 of 36 © Boardworks Ltd 2004 Inheritance introduction To understand how inheritance works you need to know: the definition of certain genetic terms, and how to use genetic crosses to determine the characteristics of offspring. In this unit, petal colour is used to show how characteristics are inherited in offspring. 3 of 36 © Boardworks Ltd 2004 Homologous chromosomes In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents. chromosome from female parent chromosome from male parent Chromosomes are matched in pairs that contain one chromosome inherited from each parent. So are the genes in a matching pair of chromosomes exactly the same? 4 of 36 © Boardworks Ltd 2004 Different versions of genes The chromosomes in a matching pair contain the same type of genes that code for the same characteristics. gene for petal colour gene for petal colour version for red petals version for yellow petals Each chromosome may have a different version of a gene. Different versions of a gene, that code for different versions of a characteristic, are called alleles. 5 of 36 © Boardworks Ltd 2004 Pairs of alleles – homozygous If the alleles in a matching pair are the same, they are called homozygous alleles. allele for yellow petals allele for yellow petals allele for red petals allele for red petals What colour are the flowers with these homozygous pairs of alleles? 6 of 36 (Click twice on each bud to reveal the flower; click again to close them.) © Boardworks Ltd 2004 Pairs of alleles – heterozygous If the alleles in a matching pair are different, they are called heterozygous alleles. allele for red petals (Click twice on the bud to reveal the flower; click again to close it.) allele for yellow petals Which characteristic is expressed if alleles are different? Some alleles are dominant to other forms of a gene and will always be expressed. Which is the dominant allele in this heterozygous pair? Which is the recessive allele in this heterozygous pair? 7 of 36 © Boardworks Ltd 2004 Representing alleles Letters are used to represent different alleles. A dominant allele is always a capital letter. allele for red petals = R A recessive allele is always the corresponding small letter. allele for yellow petals = r The allele pair for each characteristic is called the genotype. What colour are flowers with the genotype Rr? 8 of 36 © Boardworks Ltd 2004 Genotypes and phenotypes The allele pair for each characteristic is called the genotype. The physical expression of an allele pair is the phenotype. What are the phenotypes of these genotypes? genotype: RR rr Rr phenotype: (Click twice on each bud to reveal the flowers; click again to close them.) 9 of 36 © Boardworks Ltd 2004 Genetic jargon What do these genetic terms mean? gene Section of DNA that codes for a particular trait or characteristic. allele A different form of a gene that codes for a different version of a characteristic. genotype A description of the pair of alleles present for a characteristic. phenotype The physical expression of the alleles. 10 of 36 © Boardworks Ltd 2004 Genetic jargon What do these genetic terms mean? homozygous Pair of alleles that produce a characteristic that are the same, e.g. HH. heterozygous Pair of alleles that produce a characteristic that are different, e.g. Hh. 11 of 36 dominant An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. recessive An allele that will only be expressed when both alleles are of this type, represented by a lower case letter. © Boardworks Ltd 2004 Matching pairs – genes 12 of 36 © Boardworks Ltd 2004 Matching pairs – characteristics 13 of 36 © Boardworks Ltd 2004 Contents Inheritance All about alleles Homozygous cross Heterozygous cross Using a test cross Co-dominance 14 of 36 © Boardworks Ltd 2004 RR x rr – crossing homozygous parents What are the possible offspring of a cross between a homozygous red flower and a homozygous yellow flower? Homozygous means that both alleles of a gene are the same. Red is the dominant allele for these flowers, so the alleles for petal colour are: red = R , yellow = r. phenotype: x genotype: 15 of 36 RR rr © Boardworks Ltd 2004 RR x rr – F1 offspring parental genotype: gametes: RR R x R r r F1 offspring genotype: rr r r R Rr Rr R Rr Rr ? What are the phenotypes of the F1 offspring? 16 of 36 © Boardworks Ltd 2004 RR x rr – F1 phenotypes parental genotype: F1 genotypes: RR Rr x Rr rr Rr Rr F1 phenotypes: (Click twice on the buds to reveal the flowers; click again to close them.) The possible offspring of a cross between two homozygous parents are always heterozygous and so the dominant characteristic is always expressed in this generation. 17 of 36 © Boardworks Ltd 2004 Homozygous cross activity 18 of 36 © Boardworks Ltd 2004 Contents Inheritance All about alleles Homozygous cross Heterozygous cross Using a test cross Co-dominance 19 of 36 © Boardworks Ltd 2004 Rr x Rr – crossing heterozygous parents The offspring (Rr) from the first cross (RR x rr) are called the F1 generation. What happens in a cross between these offspring? Both parent plants are now heterozygous, so the alleles in each plant are different. F1 generation phenotype: X genotype: 20 of 36 Rr Rr © Boardworks Ltd 2004 Rr x Rr – F2 offspring parental genotype: gametes: Rr R x r R R F2 offspring genotype: Rr r r R RR Rr r Rr rr ? What are the phenotypes of the F2 offspring? 21 of 36 © Boardworks Ltd 2004 Rr x Rr – F2 phenotypes parental genotype: F2 genotypes: Rr RR x Rr Rr Rr rr F2 phenotypes: (Click twice on the buds to reveal the flowers; click again to close them.) In the F2 generation, 3 of the 4 possible offspring are red. Only one offspring shows the recessive phenotype. When two heterozygous parents are crossed, the possible offspring will always show a 3:1 ratio in favour of the dominant phenotype. 22 of 36 © Boardworks Ltd 2004 Heterozygous cross activity 23 of 36 © Boardworks Ltd 2004 Contents Inheritance All about alleles Homozygous cross Heterozygous cross Using a test cross Co-dominance 24 of 36 © Boardworks Ltd 2004 What is a test cross? A test cross allows you to find out if an organism showing a dominant characteristic is homozygous or heterozygous for the dominant allele. For example the genotype of a red flower could be: RR or Rr What could you cross a red flower with to find its genotype? 25 of 36 © Boardworks Ltd 2004 Carrying out a test cross A test cross is carried out between the flower of unknown genotype and another flower whose genotype is known. For example, a yellow flower can only have the genotype rr because it’s recessive. So the test cross is: x ? (RR or Rr) 26 of 36 rr © Boardworks Ltd 2004 Test cross – 2 types If the red flower is homozygous (RR) then the cross is the same as the first cross (RR x rr). All of the offspring will be heterozygous and have red petals. What about the other possible cross between a heterozygous red flower (Rr) and yellow flower (rr)? x ? (RR or Rr) 27 of 36 rr © Boardworks Ltd 2004 Test cross offspring parental genotype: gametes: Rr R x r r r offspring genotype: rr r r R Rr Rr r rr rr ? What are the phenotypes of these offspring? 28 of 36 © Boardworks Ltd 2004 Test cross results parental genotype: F2 genotypes: Rr Rr x Rr rr rr rr F2 phenotypes: (Click twice on the buds to reveal the flowers; click again to close them.) A cross between a heterozygous parent and a recessive parent yields different types of offspring in a 1:1 ratio. 29 of 36 © Boardworks Ltd 2004 Test cross activity 30 of 36 © Boardworks Ltd 2004 Contents Inheritance All about alleles Homozygous cross Heterozygous cross Using a test cross Co-dominance 31 of 36 © Boardworks Ltd 2004 What is co-dominance? Sometimes two alleles are equally dominant to each other. In genetics, this is called co-dominance and means that neither allele is recessive to the other allele. How does co-dominance affect the offspring of a cross? For example, let’s assume that the red allele (R) and the white allele (W) are co-dominant: x RR 32 of 36 WW © Boardworks Ltd 2004 Co-dominance cross – offspring RR parental genotype: gametes: R x R W WW W W W ? offspring genotype: R RW RW R RW RW The alleles are co-dominant so both are expressed. What will the offspring flowers look like? 33 of 36 © Boardworks Ltd 2004 Co-dominance cross – phenotypes parental genotype: offspring genotypes: RW RR RW x WW RW RW offspring phenotypes: (Click twice on the buds to reveal the flowers; click again to close them.) All the offspring flowers are pink because both the red and white alleles are expressed. 34 of 36 © Boardworks Ltd 2004 Co-dominance activity FLASH 5 – 35 of 36 Blood groups © Boardworks Ltd 2004 Inheritance multiple-choice quiz 36 of 36 © Boardworks Ltd 2004