Patterns of Inheritance Objective # 1 „ In this section we will examine how studying patterns of inheritance has Define the term “monohybrid cross”. allowed scientists to learn about some Describe one of the monohybrid of the basic laws that govern the crosses carried out by Mendel and transmission of genetic information explain how the results of these from one generation to the next. crosses led him to formulate the Law of Segregation and the Law of „ You will also learn how to use this Dominance. knowledge to predict the outcome of certain types of genetics crosses. 1 2

Objective 1 Objective 1

„ So far, we have focused on how „ However, over 50 years before scientists chromosomes get passed from cell to cell understood the role that chromosomes during the eukaryotic cell cycle, and from play in transmitting hereditary one generation to the next during information, an Austrian monk named eukaryotic life cycles. Gregor Mendel discovered the basic „ Since chromosomes contain the principles of heredity by studying the hereditary information, this shows us how pattern of inheritance for 7 different the hereditary information is transmitted traits in pea plants. from one generation to the next. 3 4

Objective 1 Objective 1 Each trait that Mendel studied existed in 2 The 7 traits that discrete forms. Mendel studied are For example, seed shown in the chart shape could be on the right. wrinkled or Each trait exists in round: 2 forms.

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1 Objective 1 Objective 1

„ Mendel was successful where many before him had failed because: ¾ he used statistics to mathematically ¾ he simplified the problem by first analyze the results of his crosses and examining just one trait at a time. help determine the general patterns of his results. ¾ he limited his study to traits that existed in 2 discrete forms. ¾ he kept quantitative data over several generations, using large sample sizes. 7 8

Objective 1 Objective 1

„ Mendel’s success was also due to a „ Mendel began with crosses where he certain amount of luck because each followed the inheritance of one trait. trait that he followed happened to be Although he didn’t know it, each trait controlled by a single pair of alleles. he studied was controlled by a single This produces the simplest possible gene locus. This type of cross is called a pattern of inheritance. monohybrid cross. „ For example, Mendel followed the inheritance of flower color, which exists in 2 discrete forms: purple and white. 9 10

Objective 1 Objective 1

„ When he crossed some pure-breeding „ Next, he allowed the F1 plants to self- purple flowered plants with some fertilize in order to produce the F2 pure-breeding white flowered plants generation. Again, the results were (this is called the parental or P unexpected: generation) he got some surprising results: ¾ Out of 929 F2 plants, he found 705 ¾ all of the offspring (called the F1 with purple flowers and 224 with white generation) had purple flowers! flowers, a ratio of 3.15 to 1.

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2 Objective 1, One of Mendel’s Monohybrid Crosses Objective 1 Mendel explained his results by proposing the following 4-part hypothesis: 1)Each individual has two “hereditary factors” controlling a given trait. The pure-breeding purple parents have 2 hereditary factors for purple flowers, and the pure-breeding white plants have 2 hereditary factors for white flowers.

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Objective 1 Objective 1

„ Today we call Mendel’s “hereditary „ The appearance of an organism is factors” alleles. Scientists use letters to called its phenotype (e.g. purple represent alleles. For example, if we use flowers), and the alleles the organism “A” to represent the purple allele and “a” has is its genotype (e.g. AA). to represent the white allele, then the pure-breeding purple plants would be AA and the pure-breeding white plants would be aa.

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Objective 1 Objective 1

„ From chromosome studies we know that parents are diploid, so there are 2 „ If the 2 alleles are identical (e.g. AA), sets of chromosomes in each cell, and the individual is homozygous for that 2 alleles at each gene locus. trait. If the 2 alleles are different (e.g. Aa), the individual is heterozygous for „ Because flower color is controlled by that trait. one gene locus, each parent must have 2 alleles controlling this trait.

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3 Objective 1 Objective 1 2) When the parents produce gametes, „ Today we know that homologous the 2 hereditary factors separate, and chromosomes separate during meiosis I, each gamete receives one of the 2 leading to formation of haploid gametes. factors. Therefore, all gametes „ Because gametes are haploid, each produced by the purple parent (AA) gamete has 1 set of chromosomes, and 1 have one purple allele (A), and all allele at every gene locus. gametes produced by the white parent „ Because flower color is controlled by one (aa) have 1 white allele (a). This is gene locus, each gamete must have 1 called Mendel’s Law of Segregation. 19 allele controlling this trait. 20

Objective 1 Objective 1 3) The offspring are formed when a „ Today we know the offspring are gamete from one parent joins with a diploid, so there are 2 sets of gamete from the other parent. chromosomes in each cell, and 2 alleles

Therefore, each F1 offspring receives at each gene locus (one inherited from one purple hereditary factor (A) each parent). from the purple parent (AA) and one white hereditary factor (a) from the white parent (aa).

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Objective 1 Objective 1 4) When an individual is heterozygous, „ One the next slide, you can see how only one of the 2 alleles is expressed. Mendel’s Laws of Inheritance can be Mendel called the expressed allele used to correctly predict the outcome

dominant, and the non-expressed of both the F1 and the F2 generations: allele recessive. Because purple is

dominant to white, all the F1 plants (Aa) have purple flowers. This is known as Mendel’s Law of dominance. 23 24

4 Objective 1, One of Mendel’s Monohybrid crosses Objective # 2

Explain how a testcross can be used to determine whether an individual with a dominant phenotype is homozygous or heterozygous.

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Objective 2 Objective 2

„ To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:

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Objective # 3 Objective 3

„ In a dihybrid cross, we follow alleles at 2 Define the term “dihybrid cross”. gene loci simultaneously: Describe one of the dihybrid ¾ the parents are diploid, so there are 2 crosses carried out by Mendel and alleles at each gene locus = 4 alleles total explain how the results of these ¾ the gametes are haploid, so there is 1 crosses led him to formulate the allele at each gene locus = 2 alleles total Law of Independent Assortment. ¾ the offspring are diploid, so there are 2 alleles at each gene locus = 4 alleles total 29 30

5 Objective 3 Objective 3

„ For example, in pea plants seed shape is „ With his monohybrid crosses, Mendel controlled by one gene locus where determined that the 2 alleles at a single round (R) is dominant to wrinkled (r) gene locus segregate when the gametes while seed color is controlled by a are formed. different gene locus where yellow (Y) is „ With his dihybrid crosses, Mendel was dominant to green (y). interested in determining whether alleles „ Mendel crossed 2 pure-breeding plants: at 2 different gene loci segregate one with round yellow seeds and the dependently or independently. other with green wrinkled seeds. 31 32

Objective 3 Objective 3, Dependent Assortment R R r r „ Dependent segregation (assortment) Parents Y Y y y means alleles at the 2 gene loci R r segregate together, and are transmitted Parental Gametes Y y as a unit. Therefore, each plant would F Offspring R r only produce gametes with the same 1 Y y combinations of alleles present in the F Offspring’s Gametes R r gametes inherited from its parents: 1 Y y What is the expected

33 phenotypic ratio for the F2? 34

Objective 3 Objective 3

F2 with dependent assortment: „ Independent segregation (assortment) R r means alleles at the 2 gene loci Y y segregate independently, and are NOT R R R R r transmitted as a unit. Therefore, each Y Y Y Y y plant would produce some gametes R r r r with allele combinations that were not r present in the gametes inherited from Y y y y y its parents:

Ratio is 3 round, yellow : 1 wrinkled, green 35 36

6 Objective 3, Independent Assortment Objective 3 R R r r F2 with independent assortment: Parents Y Y y y RY Ry rY ry R r Parental Gametes RR RR Rr Rr Y y RY R r YY Yy YY Yy F1 Offspring RR RR Rr Rr Y y Ry Yy Yy Yy yy F1 Offspring’s R R r r Rr Rr rr rr Gametes rY Y y Y y YY Yy YY Yy Rr Rr rr rr What is the expected ry Yy yy Yy yy phenotypic ratio for the F2? 37 Phenotypic ratio is 9 : 3 : 3 : 1 38

Objective 3 Objective # 4

„ Mendel’s dihybrid crosses showed a Explain why some genes do 9:3:3:1 phenotypic ratio for the F2 generation. NOT assort independently. „ Based on these data, he proposed the Also explain how an experiment Law of Independent Assortment, by Morgan originally which states that when gametes form, demonstrated this. each pair of hereditary factors (alleles) segregates independently of the other pairs. 39 40

Objective 4 Objective 4

„ In the early 20th century, Thomas „ At another gene locus controlling wing Hunt Morgan studied patterns of length, he found a dominant allele for inheritance for many traits in the fruit normal wings (N) and a recessive allele fly, Drosophila. for short wings (n). „ At one gene locus controlling eye „ Morgan crossed a female heterozygous color, he found a dominant allele for at both gene loci with a male red eyes (R) and a recessive allele for homozygous for both recessive alleles. purple eyes (r). Based on Mendel’s hypotheses, what is the expected outcome? 41 42

7 Objective 4 Objective 4 „ „ Genotype of parents: Offspring: ¾ ♀ Rr Nn X ♂ rr nn genotype RrNn Rrnn rrNn rrnn „ Genotype of gametes: phenotype red, red, purple, purple, normal short normal short ¾ ♀ (RN) (Rn) (rN) (rn) X ♂ (rn) expected 25% 25% 25% 25% „ Genotype of offspring: observed 47% 5% 5% 43% RN Rn rN rn „ Why don’t the observed results agree with rn RrNn Rrnn rrNn rrnn the predicted results? „ Chance or Mendel’s hypothesis is wrong. 43 44

Objective 4 Objective 4 RN Rn rN rn „ A statistical test gives a p< 0.01. What rn RrNn Rrnn rrNn rrnn should we conclude? expected 25% 25% 25% 25% ¾ There is a very small probability the observed 47% 5% 5% 43% difference was caused by chance alone. „ Female gametes are mostly RN or rn. Why „ So, what’s wrong with the hypothesis? didn’t we get 25% of RN, Rn, rN, and rn? „ R and N are linked together on one chromosome, while r and n are linked together on the homologous chromosome. 45 46

Objective 4 Objective 4

„ Unlinked genes (located on different „ Linked genes (located on the same chromosomes) assort independently: chromosome) do not assort independently:

R r R r Y y N n

R R r r R R r r N n N n Y y Y y 47% 5% 5% 43% 25% 25% 25% 25% 47 48

8 Objective # 5 Objective 5

„ Linked genes (on the same chromosome) Explain how the process of do NOT assort independently: crossing over leads to recombinant Parental type R r Parental type types. Explain how the frequency N n of recombinant types is used by geneticists to construct R R r r chromosome maps. N n N n 47% 5% 5% 43%

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Objective 5 Objective 5

„ Recombinant types are produced only „ Because crossovers occur along the when a crossover occurs between the 2 length of a chromosome at random, genes being studied: the closer 2 genes are, the smaller the A a a A A a A a chance a crossover will occur between them, and the lower the frequency of recombinant types. B b B b B b B b „ Therefore, the frequency of recombination can be used to construct genetic maps. Recombinant types No recombinant types 51 52

Objective 5 Objective # 6

„ One map unit is defined as the distance Explain how each of the following patterns between 2 genes that will produce 1% of inheritance represents a modification of recombinant types. Mendel’s original principles: „ What is the distance in map units between a) sex linkage e) polygenic traits the 2 gene loci Morgan studied: b) incomplete f) epistasis RrNn Rrnn rrNn rrnn dominance g) pleiotropy 47% 5% 5% 43% c) codominance h) environmental d) multiple alleles effects on gene ¾10 map units expression 53 54

9 Objective 6a Objective 6a

a) Sex linkage: ¾ Even though the sex chromosomes ¾ Sex in humans (and many other pair during synapsis, they are not organisms, but not all organisms) is homologous. The larger chromosome determined by genes located on a is called the X and the smaller is the Y. pair of chromosomes called the sex ¾ In humans, XX is female and XY is chromosomes. male. ¾ all other chromosomes are called ¾ What is the expected sex ratio among autosomes. the offspring when we cross a male

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Objective 6a Objective 6a

Parents XY XX ¾ Other traits, besides sex, are controlled Parental by genes on the sex chromosomes. X Y X X Gametes These are called sex-linked traits. ¾ Traits controlled by the X are X-linked. X Y F1 Offspring: ¾ Traits controlled by the Y are Y-linked. X ¾ Since most sex-linked traits are XX XY controlled by the X, you can assume X-linkage, unless it says Y-linked. X XX XY 57 58

Objective 6a Objective 6a

¾ X-linked traits are an exception to Parents XHY XHXh Mendel’s laws because females have 2 Parental alleles for each X-linked trait, but males XH Y XH Xh Gametes have only 1. ¾ In humans, hemophilia is caused by a XH Y F1 Offspring: recessive allele on the X chromosome. XH Two normal parents have a son with XHXH XHY hemophilia. What is the probability their next child will also have hemophilia? Xh XHXh XhY 59 60

10 Objective 6b Objective 6b b) Incomplete dominance: ¾ neither allele is dominant and heterozygous individuals have an intermediate phenotype ¾ for example, in Japanese four o’clock, plants with one red allele and one white allele have pink flowers:

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Objective 6c Objective 6d c) Codominance: d) Multiple alleles: ¾ neither allele is dominant and both ¾ when there are more than 2 possible alleles are expressed in heterozygous alleles at a given gene locus (even individuals though each diploid individual has ¾ we will examine an example of only 2). codominance when we discuss ¾ the human gene locus that controls human ABO blood types ABO blood type involves multiple alleles and codominance.

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Objective 6d Objective 6d

¾ This gene (designated I) codes for an ¾ These sugars act as recognition enzyme that adds sugar molecules to markers (antigens) for the immune lipids on the surface of red blood cells. system. ¾ There are 3 possible alleles at this gene ¾ The immune system will produce locus: antibodies against cells with foreign A I adds galactosamine antigens and mark them for IB adds galactose destruction. i adds neither sugar 65 66

11 Objective 6d, ABO Blood Type Objective 6d Antigens present Blood Possible genotypes Type ¾ If a person with type AB blood galactosamine A IA IA IA i marries a person with type O, what only blood types are possible among the galactose B IB IB IBi offspring? only both AB IAIB neither O ii

IA and IB are codominant 67 68

Objective 6d Objective 6e

Parents IAIB ii e) Polygenic traits: Parental ¾ most traits are not controlled by a IA IB i i Gametes single gene locus, but by the combined interaction of many gene i i loci. These are called polygenic traits. F1 Offspring: ¾ polygenic traits often show IA IAi IAi continuous variation, rather then a few discrete forms: IB IBi IBi 69 70

Objective 6e Objective 6f f) Epistasis: ¾ this is a type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus. ¾ for example, in Labrador retrievers one gene locus affect coat color by controlling how densely the pigment 71 eumelanin is deposited in the fur. 72

12 Objective 6f Objective 6f

¾ a dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat ¾ however, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow fur no matter which alleles are at the first locus: 73 74

Objective 6g Objective 6h g) Pleiotropy: h) Environmental effects on gene ¾ this is when a single gene locus expression: affects more than one trait. ¾ the phenotype of an organism ¾ for example, in Labrador retrievers depends not only on which genes it the gene locus that controls how has (genotype), but also on the dark the pigment in the hair will be environment under which it also affects the color of the nose, develops. lips, and eye rims.

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Objective 6h Objective 6h

¾ Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.

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