Mendel: the rogue monk But NOT bulletproof…

Mendelian Genetics Your knowledge goals for the day: Who mendel was and what he did Mendel’s laws and how they relate to meiosis Difference between genotype and phenotype How to use a punnett square to find the frequencies among offspring

What he did In the mid 1850’s Mendel began breeding plants to study variation and inheritance He used peas, which turned out to be a GREAT idea: Easy to control mating Easily observable variation Generation time is relatively short

What he Found Mendel found that when crossing two true-breeding plants of different phenotype (the P generation), the Filial 1 (F1) generation was all of ONE phenotype. However, when F1 plants were ‘self-crossed’ he found that the F2 generation contained plants of BOTH phenotypes in a 3:1 ratio.

Talking the Talk (aka Vocabulary is awesome) Know these words: Phenotype: what something looks like Genotype: the actual genetic makeup Allele: alternate version of a gene Homozygous: two identical alleles Heterozygous: two different alleles Dominant: Expressed in the heterozygote Recessive: Masked in the heterozygote True breeding: homozygous! Testcross: crossing with a homozygous recessive individual to reveal genotype

His Conclusions Mendel deduced that there was ‘something’ ‘being passed’ on that allowed for alternate versions of genes (the two phenotypes). He reasoned that since a ‘blending’ of phenotypes was NOT seen, these alternate versions must be distinct entities and acting in a dominant or recessive fashion. He called these alternate versions alleles.

b B A locus is a point on the chromosome where a gene is located Alleles are located on homologous chromosomes b B

Alleles –different forms of the same gene

Genotype Pairs of genes that an individual has for a certain characteristic EX: GG, Gg, gg Phenotype The physical appearance of an individual

Dominant Gene A gene that masks the effect of another gene in the heterozygous condition. Recessive Gene A gene whose effect is masked by a dominant gene in the heterozygous condition

Mendel got the idea of Dominance/Recessive from the F1 generation Mendel got the idea of Independent Assortment from the F2 generation

A Testcross

Use a punnett square – all the cool kids are doing it!

Suppose in flowers R, for red flowers is dominant to r, for white. Genotype Phenotype

Suppose a homozygous red and a white were crossed Suppose a homozygous red and a white were crossed. If there were 200 offspring in the F2 generation, how many would be white?

Separation

A Testcross

A Punnett Square: use it!

An Example A tall plant and a short plant are crossed. ALL the offspring are tall plants. These offspring are self-crossed and the next generation reveals a 3:1 ratio of Tall: Short plants.

His Conclusions Mendel deduced that there was ‘something’ ‘being passed’ on that allowed for alternate versions of genes (the two phenotypes). He reasoned that since a ‘blending’ of phenotypes was NOT seen, these alternate versions must be distinct entities and acting in a dominant or recessive fashion. He called these alternate versions alleles.

In gorillas the gene for black fur, B is dominant to the gene for white, b (albino) fur Genotype Phenotype BB Bb bb

In gorillas the gene for black fur, B is dominant to the gene for white, b (albino) fur Genotype Phenotype BB Black Homozygous Black Bb Heterozygous Black bb White

Phenotypic Ratio= 100% Black BB Genotypic Ratio= 100% BB Phenotypic Ratio= 100% Black

Genotypic Ratio= 2 : 2 Bb:bb Phenotypic Ratio= 2 : 2 Black:White B b 2 : 2 Black:White

Phenotypic Ratio=100% Black Bb Genotypic Ratio= 100% Bb Phenotypic Ratio=100% Black

Phenotypic Ratio=100% White b bb Genotypic Ratio= 100% bb Phenotypic Ratio=100% White

B BB b Bb Genotypic Ratio= 2 : 2 BB:Bb Phenotypic Ratio= 100% Black

Genotypic Ratio= 1 : 2 : 1 BB:Bb:bb Phenotypic Ratio= 3 : 1 3 : 1 Black:White

A couple who are both carriers of the gene for cystic fibrosis have two children who have cystic fibrosis. What is the probability that their next child will have cystic fibrosis? 0% 25% 50% 75% 100% Answer: 2 Source: Barstow - Test Bank for Biology, Seventh Edition, Question #6

A couple who are both carriers of the gene for cystic fibrosis have two children who have cystic fibrosis. What is the probability that their next child will be phenotypically normal? 0% 25% 50% 75% 100% Answer: 4 Source: Barstow - Test Bank for Biology, Seventh Edition, Question #7

Blood Groups: an example of codominance

Antigen on Antibody blood cell in plasma anti A & anti B A & B Phenotype Genotypes blood cell in plasma A A anti B anti A & O None anti B AB A & B None B B anti A

A B B Anti-B antibody A Anti-A antibody A Anti-B antibody B Anti-A antibody

Antigen on Antibody blood cell in plasma anti A & anti B A & B Phenotype Genotypes blood cell in plasma A A anti B anti A & O None anti B AB A & B None B B anti A

Antigen on Antibody blood cell in plasma anti A & anti B A & B Phenotype Genotypes blood cell in plasma A A anti B anti A & O None anti B AB A & B None B B anti A

Three babies were recently mixed up in a hospital Three babies were recently mixed up in a hospital. After consideration of the data below, which of the following represent the correct baby/parent combinations? Couple # Blood groups I A and A II A and B III B and O Baby # 1 B 2 O 3 AB I-3, II-1, III-2 I-1, II-3, III-2 I-2, II-3, III-1 I-2, II-1, III-3 I-3, II-2, III-1 Answer: 3 Source: Barstow - Test Bank for Biology, Seventh Edition, Question #49

OMG HELP! How do I do this? Pedigree Analysis Revealing Mendelian patterns

What is the probability that individual C-1 is Ww? 3/4 1/4 2/4 2/3 1 Answer: 5 Source: Barstow - Test Bank for Biology, Seventh Edition, Question #19

What do pedigrees do? A pedigree shows the phenotypes of one or more traits as it is passed through an extended family. Through analysis of the pedigree, you can often deduce genotypes and the pattern of inheritance (autosomal, sex linked, dominant vs. recessive)

How to Read them

A simple one The pedigree below is studying the incidence of blonde hair in a family. In humans, dark hair (B) is dominant to blonde hair (b). In this case, individuals who are shaded in are homozygous recessive. Individuals who have clear circles and squares have at least one dominant gene. What are the genotypes of persons A through F above?

Widow’s peak and earlobes!

Dominant or Recessive, also genotypes

Mode of inheritance?

If Bob’s mother has a condition known as cystic fibrosis, which is a homozygous recessive disorder, and his father does not have cystic fibrosis. Bob and his 5 brothers all come out normal, what is Bob’s fathers GENOTYPE?

If your F2 generation has 25% chance of showing a homozygous recessive If your F2 generation has 25% chance of showing a homozygous recessive. And the past crosses were all from the same batch of plants, what are the parents GENOTYPE?

Karyotype

Noninvasive techniques. Ultrasound imaging. Maternal blood contains fetal blood cells that can be tested.

Karyotype Autosome centromere Sex Chromosomes

Sex Linked Gene= a gene on the X sex chromosome Males determine the sex - half the sperm have an X and the other half has a Y Genotype Phenotype XRXR Red ♀ XRXr XrXr White ♀ XRY Red ♂ XrY White ♂ XR Y Xr XRXr XrY F1 Genotypic ratio= 2:2

1. What are the possible genotypes for eye color of a female fruit fly 1. What are the possible genotypes for eye color of a female fruit fly? Of a male? Female; XRXR, XRXr, or XrXr; male: XRY or XrY

2. Construct a cross that could produce a white-eyed female fruit fly. Students should show a cross between a white-eyed male (XrY) and a heterozygous female (XRXr) or a homozygous recessive female (XrXr). Homozygous recessive female offspring (XrXr) will have white eyes.

Sex Linked Gene= a gene on the X sex chromosome Males determine the sex - half the sperm have an X and the other half has a Y Genotype Phenotype XRXR Red ♀ XRXr XrXr White ♀ XRY Red ♂ XrY White ♂ Xr Y XR XRXr XRY XrXr XrY F2 Genotypic ratio= 1:1:1:1

1. Are attached earlobes a dominant or a recessive trait? Explain. Attached earlobes are a recessive trait. All individuals with attached earlobes are homozygous for the recessive allele.

2. What is the genotype of a colorblind male 2. What is the genotype of a colorblind male? What possible genotypes can his mother have? Explain. Colorblind male: XbY; mother: XBXb or XbXb. The mother must be a carrier of the recessive allele or colorblind herself.

Carrier – a normal female with a sex linked recessive trait Genotype Phenotype XCXC Normal ♀ XCXc Normal ♀ (carrier) XcXc Color Blind ♀ XCY Normal ♂ XcY Color Blind ♂ Carrier – a normal female with a sex linked recessive trait

Carrier= A normal female with a sex linked recessive gene.

Genotype Phenotype XRXR Normal ♀ XRXr XrXr Color Blind ♀ XRY Normal ♂      

Instructions: For each of the following pedigrees, determine if the mode of inheritance is a) autosomal dominant b) autosomal recessive c) sex-linked recessive d) sex-linked dominant. It is possible, in some cases, for a pedigree to be explained by more than one mode of inheritance.

By the law of independent assortment, each pair of alleles segregates into gametes independently Mendel’s experiments that followed the inheritance of flower color or other characters focused on only a single character via monohybrid crosses. He conduced other experiments in which he followed the inheritance of two different characters, a dihybrid cross. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape. The allele for yellow seeds (Y) is dominant to the allele for green seeds (y). The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

This was not consistent with Mendel’s results. One possibility is that the two characters are transmitted from parents to offspring as a package. The Y and R alleles and y and r alleles stay together. If this were the case, the F1 offspring would produce yellow, round seeds. The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross. This was not consistent with Mendel’s results. Fig. 14.7a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

An alternative hypothesis is that the two pairs of alleles segregate independently of each other. The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait. In our example, the F1 offspring would still produce yellow, round seeds. However, when the F1’s produced gametes, genes would be packaged into gametes with all possible allelic combinations. Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A Dihybrid Cross

Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ration in the F2 generation. Each character appeared to be inherited independently. The independent assortment of each pair of alleles during gamete formation is now called Mendel’s law of independent assortment. One other aspect that you can notice in the dihybrid cross experiment is that if you follow just one character, you will observe a 3:1 F2 ratio for each, just as if this were a monohybrid cross.

Rule for making a Sex Cell: Choose 1 gene from each pair of genes. Ex each sex cell has one R and one Y.

Ok you try it! Work with your partners and attempt to answer this question. Feather color in budgies is determined by two different genes that affect the pigmentation of the outer feather and its core. Y_B_ is green; yyB_ is blue; Y_bb is yellow; and yybb is white. A green budgie is crossed with a blue budgie. Which of the following results is not possible? all green offspring all blue offspring all white offspring all yellow offspring All of the above are possible, but with different probabilities. Answer: 5 Source: Barstow - Test Bank for Biology, Seventh Edition, Question #35

Is all this stuff really true? Not exactly… Q: The law of independent assortment is often violated. What might make this so? A: ‘Linked genes’ (which you’ll learn about over break) – genes on the same chromosome.

Was Mendel a liar? Kind of. Mendel was a perfectionist and, when he didn’t find a ‘perfect’ 3:1 ratio, fudged his results ever so slightly to make them closer. However, his science was still brilliant and his real results were close enough that we overlook his little ‘adjustments’ and still hail him as a hero. Mendel Rules!?

Mendel is so smart, S-M-R-T By tracking TWO traits at the same time, Mendel observed that they did not necessarily ‘travel together.’ That is, they acted INDEPENDENTLY. This is the ‘Law of Independent Assortment’ – it states that during gamete production, genes will act independently of each other. Q: In what phase of Meiosis does this relate to? A: Metaphase I – alignment along the metaphase plate is random!