Lab 11 Patterns of Inheritance

Note: All your answers to questions must be in Red or other color (not including blue) for easier grading. Points will be deducted if you do not distinguish your answers. Lab 11. Patterns of Inheritance Objectives:  Perform a Monohybrid (one-trait) cross and explain how the results predict all the possible offspring  Relate Mendel’s law of segregation to the results of a Monohybrid (one-trait) cross  Explain and predict the results of a monohybrid cross in corn plants .  Perform a Dihybrid (two-trait) cross and explain how the results predict all the possible offspring  Explain and predict the results of X-Linked crosses in Drosophila Vocabulary:  Gene  Alleles  Homozygous dominant  Homozygous recessive  Heterozygous  Genotype  Phenotype  Carrier  Punnett square  Probability  Law of Segregation  Law of Independent Assortment  X-linked or sex-linked  Sex chromosomes  Autosomes Introduction: Gregor Mendel is known as the “father of genetics” due to his extensive research of inherited traits – specifically in the pea plant. Because of Mendel’s work, the contributions of many other scientists, and advances in technology we now know that diploid individuals have two copies of each gene, called alleles , which correspond to exhibited traits.

We also know that alleles are located on chromosomes. Since diploid organisms have two copies of each chromosome it is possible for them to be homozygous dominant (two dominant alleles, AA), homozygous recessive (two recessive alleles, aa), or heterozygous (one dominant and one recessive allele, Aa). These combinations of alleles are called the genotype . When we refer to the visible traits, or appearance, we are referring to the organism’s phenotype . If the phenotype associated with a given version of a gene is observed when an organism has only one copy, the allele is said to be dominant (denoted by uppercase letters, A). The phenotype will be seen whether the organism has one copy of the allele (heterozygous, Aa) or two copies of that allele (homozygous, AA). If the phenotype associated with a given version of a gene is observed only when an individual has two identical copies, the allele is said to be recessive (aa). The phenotype will be observed only when the individual is homozygous for the allele concerned. An individual with only one copy of the allele will not show the phenotype but will be able to pass the allele on to subsequent generations. As a result, an individual heterozygous for an autosomal recessive allele is known as a carrier . Mendelian Inheritance Patterns Scientists use a grid-like tool ( Punnett Square ) to make predictions about various genetic problems. The Punnett Square shows only the probability (the chance of something occurring) of what might occur and not the actual results. For example, if one wants to flip a coin 100 times, since there are 2 sides to the coin, they can expect 50 heads and 50 tails. However, if you actually flip the coin 100 times, you may actually get 60 heads and 40 tails. Punnett Squares only show the chances of what might occur each time the event is undertaken. They do not show the actual outcome. Recall that Mendel formulated the First Law of Inheritance which states that: Each organism contains two alleles for each trait (gene), and the alleles segregate (separate) during formation of gametes. Each gamete (egg or sperm) contains only one allele for each gene. Upon fertilization, the resulting offspring will have two alleles for each trait – one from each parent. Using a Punnett square , we can predict the possible genotypes and phenotypes of resulting offspring, when crossing two parents whose genotypes are known. For example: If we want to know the possible offspring genotypes from a cross between a homozygous dominant male and a homozygous recessive female, we can use a Punnett square to predict the possible outcomes.  Homozygous dominant Male = GG  Homozygous recessive female = gg

Materials:  (2) Any two-sided fair coin (heads on one side, tails on the other)  Calculator

Procedure: 1. You will pick up 2 coins. Each side represents one allele of the same gene, Heads (H) and tails (h), respectively. Since each coin has 1- heads and 1-tails it will represent a heterozygous parent. Since you will use two coins at the same time your cross is Hh x Hh. 2. To simulate a monohybrid cross, you will toss TWO coins, SIMULTANEOUSLY, each coin represents one of the heterozygous parents (Hh x Hh). 3. Record the resulting genotype from the tossed coins, which side lands face up for each coin. The only possibilities that can be made from this toss are: HH (homozygous heads), Hh (heterozygous heads), or hh (homozygous tails). Mark the resulting genotype and phenotype in the data table. 4. Pick up your two coins and conduct the same process (steps 1-3) 14 more times (15 total trials). Record your data in Table 1. Results: Table 1: Monohybrid Cross Simulation – 2 two-sided coin toss Trial Offspring Genotype Offspring Phenotype 1 Hh Heterozygous heads 2 hh Homozygous tails 3 Hh Homozygous tails 4 Hh Heterozygous heads 5 Hh Heterozygous heads 6 HH Homozygous heads 7 HH Homozygous heads 8 Hh Heterozygous heads 9 Hh Heterozygous heads 10 Hh Heterozygous heads 11 HH Homozygous heads 12 Hh Homozygous tails 13 HH Homozygous heads 14 Hh Heterozygous heads 15 Hh Heterozygous heads Total number of offspring with: 2. Homozygous dominant genotype: 4 3. Heterozygous genotype: 8 4. Homozygous recessive genotype: 3

16. Looking at your Punnett square, what is the phenotypic ratio? 3:1 17. Does your phenotypic ratio from the coin toss match the ratio of your Punnett square? (You calculated this in the results section) Why or why not? No, because in a coin-toss experiment, the results are purely random. 18. In Mendel’s paper he provided data for 1000s of monohybrid crosses. How did having this large amount of data allow Mendel to arrive at his final conclusion of a phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1 for monohybrid crosses? The data collected from counting 1000s of offspring...allowed Mendel to determine the law of segregation which states that each trait should have at least two inheritable alleles, these alleles should segregate during gamete formation and at fertilization organisms again have two alleles one from each parent. 19. Was your hypothesis supported by your data? Why or why not? I believe my hypothesis was supported by the data. Although, the outcome of a random coin toss sounds foolish, it had a turnout of Mendel’s phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1 for monohybrid crosses. 20. Is there anything you could have changed about this experiment so that your hypothesis was better tested? As my toddler often tells me when we play “I spy with my little eye, something...” and I give my answer, she replies with, “not quite .”

Part 2: Monohybrid Cross – Life Example in Corn Just like Mendel’s pea plants, corn also contains genes which have dominant and recessive alleles that follow Mendel’s laws when used in monohybrid crosses. In fact, filial generation (F 2 ) multicolored corn which exhibits both purple (P) and yellow (p) kernels (seeds) results from a cross of a heterozygous purple (Pp) parent with a second heterozygous purple (Pp) parent (F 1 ). See Image 1, a monohybrid cross of PP x pp to produce F1 generation Pp offspring, which are then crossed to produce the Mendelian ratio of 3:1 and 1:2:1, genotype, and phenotype respectively. Figure 1: Monohybrid cross of homozygous purple (PP) and homozygous yellow (pp) parents. Filial generation 1 (F1) produces all purple Pp offspring. Filial generation 2 (F2) is a cross of Pp and Pp F1 offspring, which produces 3:1 genotypic ratio and 1:2:1 genotype. In this experiment you will be given images of F 2 generation corn cobs and asked to count the numbers of purple and yellow offspring produced. You will then determine if your data fits the expected 3:1 phenotypic and 1:2:1 genotypic ratio predicted by Mendel’s Law of Segregation. Question : When analyzing F 2 offspring from a true monohybrid cross using corn plants does Mendel’s 3:1 phenotypic and 1:2:1 genotypic ratio hold true? Hypothesis: If there is more purple kernels than yellow, then it suggests a dominance of genes related to the purple color since cob carries genetic traits for both colors. Materials:

Results: Table 2: Results of Monohybrid corn cobs count # of Purple Kernels # of Yellow Kernels Cob A 26 8 Cob B 25 8 Cob C 25 17 Average 25 8 Phenotypic Ratio: 3:1 Questions: 21. Did your phenotypic ratio differ from the expected phenotypic ratio? If so, explain your answer. It didn’t differ from the expected phenotypic ratio. 22. Why were you only asked to calculate the phenotypic ratio and not the genotypic ratio? Because a phenotypic ratio is a ratio comparing the possible outcomes for an organism based on physical appearance/visible traits. And the genotypic ratio is the comparison of genetic information, the frequency. 23. Draw and label a Punnett square which results in a 3:1 Phenotypic ratio of offspring. Part 3: Dihybrid Crosses – Crosses that Involve Two Traits Mendel’s studies with dihybrid crosses, two-trait crosses, showed that when two dihybrid organisms reproduced (AaBb, heterozygote for two known genes) the phenotypic ratio of the offspring was 9:3:3:1, giving four possible phenotypes. He rationalized that this was only possible if each allele pair was able to sort into gametes independent of the other allele pair. This led Mendel to his second finding, the law of independent assortment, which states that each pair of alleles separates and sorts into gametes independently of the members from another allele pair. This means

Figure 4: Dihybrid cross of RrYy self-fertilized pea plant. The resulting offspring fulfill the 9:3:3:1 ratio predicted by Mendel. Some Shortcuts : In any case where the parents are heterozygous for both traits (AaBb x AaBb), you will always get a 9:3:3:1 ratio. 9 is the number for the two dominant traits, 3 is the number for a dominant/recessive combination, and only 1 individual will display both recessive traits. Another way to determine the ratios is to do it mathematically 3/4 of all the offspring will have round seeds 3/4 of all the offspring will have yellow seeds 3/4 ∗ 3/4 ∗ 3/4 = 9/16 will have round, yellow seeds.

Crosses That Involve 2 Traits: Consider: RrYy x rryy The square is set up as shown below. You might notice that all four rows have the same genotype. In this case, you really only need to fill out the top row, because 1/4 is the same thing as 4/16

How many out of 16 have gray fur and red eyes? _8_ How many out of 16 have white fur and black eyes? _0_ How many out of 16 have white fur and red eyes? _0_ 26. Both male and female rabbits have the genotype GgBb. Determine the male gametes produced by this rabbit (the sperm would have these combinations of alleles remember to use FOIL). GB, Gb, gB, gb 27. Use the gametes from #26 to set up a Punnett square below. Put the male's gametes on the top and the female's gametes down the side. Then, fill out the square and determine what kind of offspring would be produced from this cross and in what proportion. Online classes will need to draw the Punnett square on a piece of paper and upload a picture of their completed answer. 28. An aquatic arthropod called a Cyclops has antennae that are either smooth or barbed. The allele(gene)for barbs is dominant. In the same organism, resistance to pesticides is a recessive trait. Make a "key" to show all the possible genotypes (and phenotypes) of this organism. Use the rabbit key to help you if you're lost. Genotype BB= barbs antennae; not resistant to pesticides Bb= barbs antennae; not resistant to pesticides bb= smooth antennae; resistant to pesticides RR= barbs antennae; not resistant to pesticides Rr= barbs antennae; resistant to pesticides rr= smooth antennae; resistant to pesticides Phenotype Bbrr=barbed antennae, resistant to pesticide bbRR=smooth antennae, resistant to pesticide bbrr=smooth antennae, resistant to pesticides BbRr=barbed antennae, resistant to pesticide

29. A Cyclops that is resistant to pesticides and has smooth antennae is crossed with one that is heterozygous for both traits. What are the genotypes of the parents? bbrr x RRbb 30. Set up a Punnett square for the cross and show the phenotypic ratios. Online classes will need to draw the Punnett square on a piece of paper and upload a picture of their completed answer. Part 4: X-linked Genes Introduction: In diploid organisms, each body cell (or 'somatic cell') contains two copies of the genome. So each somatic cell contains two copies of each chromosome and two copies of each gene. The exceptions to this rule are the sex chromosomes that determine sex in a given species. For example, in the XY system that is found in most mammals—including human beings— males have one X chromosome and one Y chromosome (XY) and females have two X chromosomes (XX). The paired chromosomes that are not involved in sex determination are called autosomes , to distinguish them from the sex chromosomes. Human beings have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (X and Y). Within a population, there may be several alleles for a given gene. Individuals that have two copies of the same allele are referred to as homozygous for that allele; individuals that have copies of different alleles are known as heterozygous for that allele. The inheritance patterns observed will depend on whether the allele is found on an autosomal chromosome or a sex chromosome , and on whether the allele is dominant or recessive . In many organisms, the determination of sex involves a pair of chromosomes that differ in length and genetic content - for example, the XY system used in human beings and other mammals. The X chromosome carries hundreds of genes, and many of these are not connected with the determination of sex. The smaller Y chromosome contains several genes responsible for the initiation and maintenance of

 red-eyed, male: X R Y 33. Show the cross of a white-eyed female X r X r with a red-eyed male X R Y. Online classes will need to draw the Punnett square on a piece of paper and upload a picture of their completed answer. 34. Show a cross between a pure, red-eyed female and a white-eyed male. Online classes will need to draw the Punnett square on a piece of paper and upload a picture of their completed answer. 35. What are the genotypes of the parents? Male (red-eyed): XRY; Female (white eyes ) : X r X r (relating to Q33) Male (white eyed) : X r Y; Female (red eyes): XRX r (relating to Q34) 36. How many of the offspring are:  white-eyed, male: 0  white-eyed, female: 0  red-eyed, male: 2 37. red-eyed, female: Show the cross of a red-eyed female (heterozygous) and a red-eyed male. Online classes will need to draw the Punnett square on a piece of paper and upload a picture of their completed answer.

38. What are the genotypes of the parents? The genotypes are XRYR and XRY 39. How many of the offspring are:  white-eyed, male: 0  white-eyed, female: 0  red-eyed, male: 1  red-eyed, female: 1 Licenses and Attributions:  " Inheritance" by Susan Burran and David DesRochers, LibreTexts is licensed under CC BY-SA.  Laws of Inheritance. (2021, March 23). Retrieved July 23, 2021, from https://bio.libretexts.org/@go/page/50387