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BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 1: MONOHYBRID CROSSES: - a cross between 2 organisms which differ in only one trait PROBLEM: In man, dark hair is dominant to blonde hair. When a blonde woman marries a heterozygous, dark haired man, what will be the genotypic and phenotypic ratios of their children? STEPS 1. determine the trait and assign letters ANSWER Dark = D Blonde = d 2. write phenotypes and genotypes of parents Man Dark hair x Woman blonde hair Dd dd 3. determine the gametes (egg, sperm) D and d d and d 4. draw the punnet square 5. write the genotype and phenotypes and ratios of the offspring (F1 generation) d d D Dd Dd d dd dd F1 Genotypes Genotypic ratios F1 Phenotypes Phenotypic ratios Dd 2/4 = 50% Dark hair 2/4 = 50% dd 2/4 = 50% Blonde hair 2/4 = 50% NOTE: the genotypic and phenotypic ratios may not be the same! ___________________________________________________________________________________________ GENEXAMPLE PAGE 1 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 2: DIHYBRID CROSSES: - a cross between two organisms which differ in TWO traits - ** each gamete must contain one allele from each trait ** PROBLEM: In guinea pigs, black colour is dominant to brown and short hair is dominant to long hair. If a heterozygous black, short haired guinea pig is crossed with another of the same type, what will be the genotypes, phenotypes and ratios of the offspring? Black -- B brown -- b Short -- S long -- s P phenotypes Heterozygous black, short X Heterozygous black short P genotypes BbSs X BbSs gametes: BS bS Bs bs BS bS Bs bs Punnet Square: BS bS Bs bs BS BBSS BbSS BBSs BbSs bS BbSS bbSS BbSs bbSs Bs BBSs BbSs BBss Bbss bs BbSs bbSs Bbss bbss F1 Genotypes ** Genotypic ratios F1 Phenotypes Phenotypic ratios BBSS 1 BbSS 2 Black, short 9 BBSs 2 BbSs 4 bbSS 1 bbSs 2 Brown, short 3 Bbss 1 Bbss 2 Black, long 3 bbss 1 Brown, long 1 (** group genotypes together which have the same phenotypes) ___________________________________________________________________________________________ GENEXAMPLE PAGE 2 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 3: INCOMPLETE DOMINANCE: for some traits neither allele is completely dominant or recessive when the alleles are together in the heterozygous state, both will be partially expressed, resulting in a blend or intermediate phenotype. use TWO DIFFERENT CAPITAL LETTERS to represent each trait eg. In Snapdragon flowers, Red (R) is incompletely dominant to White (W) flowers. P phenotypes X P genotypes X gametes: W W F1 Genotypes F1 Phenotypes R F1 R F1 Cross: P phenotypes X P genotypes X gametes: R W F2 Genotypes F2 Phenotypes R F2 W ___________________________________________________________________________________________ GENEXAMPLE PAGE 3 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 4: CODOMINANCE AND MULTIPLE ALLELES: some human traits are controlled by more than 2 alleles at a single locus this is called multiple allele inheritance more than 2 phenotypes are possible often these alleles are not completely dominant or recessive CODOMINANCE = both alleles are dominant and therefore BOTH are expressed completely when present I the heterozygous state. NO blending of traits; instead, both traits appear eg. Codominance red X white = red + white human blood types show multiple allele inheritance with codominance there are 3 alleles IA = red blood cell carries an A marker on its cell membrane IB = red blood cell carries an B marker on its cell membrane i = red blood cell carries NO marker on its cell membrane IA and IB are codominant and i is recessive the combinations of these alleles gives 4 blood types: GENOTYPES PHENOTYPES IA IA OR I Ai Type A blood IB IB OR I Bi Type B blood IA IB Type AB blood ii Type O blood PROBLEM: What will be the genotypes and phenotypes of the children if their mother has heterozygous Type A blood and father has heterozygous Type B? P phenotypes Heterozygous Type B P genotypes IBi IB gametes: X Heterozygous Type A IAi IA i i Punnet Square: F1 IA i F1 Genotypes F1 Phenotypes IB IAIB IBi IAIB 1/4 Type AB 25% i IAi ii IAi 1/4 Type A 25% IBi 1/4 Type B 25% ii 1/4 Type O 25% ___________________________________________________________________________________________ GENEXAMPLE PAGE 4 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 5: SEX-LINKED GENETICS: since the X chromosome is larger than the Y, it carries some genes for non-sex traits as well as those for femaleness these traits are inherited with the X chromosome and are therefore called “SEX-LINKED” traits. There are > 100 sex-linked traits (see p 746, fig 24-5) sex-linked alleles are represented as letters superscript to “X eg. Full hairline (dominant, sex-linked) = X B male pattern baldness (recessive, sex-linked) = X b NOTE: sex-linked allele is never found on a Y chromosome sex-linked traits occur more often in men than women because women have 2 X chromosomes ( the second X usually carries the dominant allele and masks the recessive one) men experience sex-linked traits more often because they only have one X, which if its carrying the recessive allele, it will be expressed (Y has no compensating dominant allele) eg. female carrier female bald B b X X inherits on dominant allele B (X ) is not bald is a carrier because she can b pass on (X ) to offspring but does not exhibit baldness male bald b b X X in order for her to be bald, b she must inherit (X ) from her b father and (X ) from her mother (rare) b X Y recessive baldness allele expressed because Y does not carry a compensating normal allele (B) NOTE: all males inherit baldness from their mothers on her X chromosome (the father donates a Y to his son, which does not carry baldness) each male who receives a Xb will be bald; only females who receive two XbXb will be bald, so baldness is more common in men a mother may pass the trait to her daughter so that she becomes a carrier and then may pass it to the grandsons NOTE: there is no such thing as a male carrier; the male either has the trait (bald) or doesn’t (not bald) about 60 sex-linked diseases are known including: baldness, colour-blindness, hemophilia (bleeder’s disease) and Duchenne Muscular Dystrophy most of these are sex-linked recessive alleles carried on the X chromosome ** when solving sex-linked problems, ALWAYS USE “X”” AND “Y” chromosomes ___________________________________________________________________________________________ GENEXAMPLE PAGE 5 OF 12 SEX-LINKED PROBLEM: HEMOPHILIA Hemophilia is a recessive sex-linked disorder in which the blood is slow to clot or does not clot at all due to a lack of clotting factors in the blood. If cut or bruised, hemophiliacs bleed profusely externally and internally and may die from blood loss. Queen Victoria of Britain passed this trait through her family. The possible genotypes and phenotypes are: female normal female carrier H H female hemophiliac (rare) H h X X h h X X X X male normal male hemophiliac H X Y h X Y If a hemophilia carrier female and a normal male marry, what is the probability that their children will be hemophiliacs? Hemo carrier female X normal male H h H X X X H X Y X h X H XH Y XH X HX H XHY Xh X HX h X hY Y F1 genotypes F1 phenotypes XHXH normal female XHXh normal carrier female XHY normal male X hY hemophiliac male ___________________________________________________________________________________________ GENEXAMPLE PAGE 6 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 1: MONOHYBRID CROSSES: - a cross between 2 organisms which differ in only one trait PROBLEM: In man, dark hair is dominant to blonde hair. When a blonde woman marries a heterozygous, dark haired man, what will be the genotypic and phenotypic ratios of their children? STEPS 1. determine the trait and assign letters ANSWER 2. write phenotypes and genotypes of parents 3. determine the gametes (egg, sperm) 4. draw the punnet square 5. write the genotype and phenotypes and ratios of the offspring (F1 generation) F1 Genotypes Genotypic ratios F1 Phenotypes Phenotypic ratios NOTE: the genotypic and phenotypic ratios may not be the same! ___________________________________________________________________________________________ GENEXAMPLE PAGE 7 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 2: DIHYBRID CROSSES: - a cross between two organisms which differ in TWO traits - each gamete must contain one allele from each trait PROBLEM: In guinea pigs, black colour is dominant to brown and short hair is dominant to long hair. If a heterozygous black, short haired guinea pig is crossed with another of the same type, what will be the genotypes, phenotypes and ratios of the offspring? P phenotypes Heterozygous black, short X Heterozygous black short P genotypes gametes: Punnet Square: F1 Genotypes Genotypic ratios F1 Phenotypes Phenotypic ratios (** group genotypes together which have the same phenotypes) ___________________________________________________________________________________________ GENEXAMPLE PAGE 8 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 3: INCOMPLETE DOMINANCE: for some traits neither allele is completely dominant or recessive when the alleles are together in the heterozygous state, both will be partially expressed, resulting in a blend or intermediate phenotype. use TWO DIFFERENT CAPITAL LETTERS to represent each trait eg. In Snapdragon flowers, Red (R) is incompletely dominant to White (W) flowers. P phenotypes Pure Red X Pure White P genotypes gametes: F1 Genotypes F1 Phenotypes F1 F1 Cross: P phenotypes PINK X PINK P genotypes gametes: F2 Genotypes F2 Phenotypes F2 ___________________________________________________________________________________________ GENEXAMPLE PAGE 9 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 4: CODOMINANCE AND MULTIPLE ALLELES: some human traits are controlled by more than 2 alleles at a single locus this is called multiple allele inheritance more than 2 phenotypes are possible often these alleles are not completely dominant or recessive CODOMINANCE = both alleles are dominant and therefore BOTH are expressed completely when present I the heterozygous state. NO blending of traits; instead, both traits appear eg. Codominance red X white = red + white human blood types show multiple allele inheritance with codominance there are 3 alleles IA = red blood cell carries an A marker on its cell membrane IB = red blood cell carries an B marker on its cell membrane i = red blood cell carries NO marker on its cell membrane IA and IB are codominant and i is recessive the combinations of these alleles gives 4 blood types: GENOTYPES PHENOTYPES Type _____ blood Type _____ blood Type _____ blood Type _____ blood PROBLEM: What will be the genotypes and phenotypes of the children if their mother has heterozygous Type A blood and father has heterozygous Type B? P phenotypes Heterozygous Type B X Heterozygous Type A P genotypes gametes: Punnet Square: F1 Genotypes F1 Phenotypes F1 ___________________________________________________________________________________________ GENEXAMPLE PAGE 10 OF 12 BIOLOGY SBI 3A0 UNIT: GENETICS TOPIC: SAMPLE GENETICS PROBLEMS ___________________________________________________________________________________________ TYPE 5: SEX-LINKED GENETICS: since the X chromosome is larger than the Y, it carries some genes for non-sex traits as well as those for femaleness these traits are inherited with the X chromosome and are therefore called “SEX-LINKED” traits. There are > 100 sex-linked traits (see p 746, fig 24-5) sex-linked alleles are represented as letters superscript to “X eg. Full hairline (dominant, sex-linked) = X B male pattern baldness (recessive, sex-linked) = X b NOTE: sex-linked allele is never found on a Y chromosome sex-linked traits occur more often in men than women because women have 2 X chromosomes ( the second X usually carries the dominant allele and masks the recessive one) men experience sex-linked traits more often because they only have one X, which if its carrying the recessive allele, it will be expressed (Y has no compensating dominant allele) eg. female carrier female bald B b X X inherits on dominant allele B (X ) is not bald is a carrier because she can b pass on (X ) to offspring but does not exhibit baldness male bald b b X X in order for her to be bald, b she must inherit (X ) from her b father and (X ) from her mother (rare) b X Y recessive baldness allele expressed because Y does not carry a compensating normal allele (B) NOTE: all males inherit baldness from their mothers on her X chromosome (the father donates a Y to his son, which does not carry baldness) each male who receives a Xb will be bald; only females who receive two XbXb will be bald, so baldness is more common in men a mother may pass the trait to her daughter so that she becomes a carrier and then may pass it to the grandsons NOTE: there is no such thing as a male carrier; the male either has the trait (bald) or doesn’t (not bald) about 60 sex-linked diseases are known including: baldness, colour-blindness, hemophilia (bleeder’s disease) and Duchenne Muscular Dystrophy most of these are sex-linked recessive alleles carried on the X chromosome ** when solving sex-linked problems, ALWAYS USE “X”” AND “Y” chromosomes ___________________________________________________________________________________________ GENEXAMPLE PAGE 11 OF 12 SEX-LINKED PROBLEM: HEMOPHILIA Hemophilia is a recessive sex-linked disorder in which the blood is slow to clot or does not clot at all due to a lack of clotting factors in the blood. If cut or bruised, hemophiliacs bleed profusely externally and internally and may die from blood loss. Queen Victoria of Britain passed this trait through her family. The possible genotypes and phenotypes are: female normal female carrier H H female hemophiliac (rare) H h X X h h X X male normal male hemophiliac H X Y X X h X Y If a hemophilia carrier female and a normal male marry, what is the probability that their children will be hemophiliacs? Hemo carrier female F1 genotypes X normal male F1 phenotypes ___________________________________________________________________________________________ GENEXAMPLE PAGE 12 OF 12
