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