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S E X and G E N E S Recitation 10 JD Price Multiplicity… Asexual reproduction •Genetic information is copied verbatim so parent and offspring has same code •Offspring and parent are clones •Adaptation requires modification of the code by organism Sexual reproduction •Genetic information is halved and combined with another cell with halved information •Unique combination of genetic information to offspring •Adaptation results when certain combinations are favorable Humans have 46 chromosomes. Shown are 1-22, plus X & Y Women: double 1-22 and X Men: double 1-22 and single X and Y Each double (homologous pair) contains similar, but not identical information Each chromosome consists of DNA coiled around and wrapped by proteins. Each side of the chromosome contains the same sequence (it’s a copy). Let’s just follow genes 1-4. The parent cell contains diploid pairs. The blue chromosome in each pair contains same trait controls but different coding. In Meiosis, the process of making sex cells, pairs are split and then each chromosome is split, rendering two pair of genetically identical sex cells (haploid). In sexual reproduction, one of the haploid cells combines with another haploid cell. In many cases, the second cell is from another individual. Two homologous pairs are brought together (their information is then duplicated). In reality, sometimes portions of the chromosome pairs are swapped – this is called “cross-over,” and complicates the picture of heredity even further. This is just one generation, with the next generation, the chance of deviation from the grandparent is even greater. Chance Because of the variety of combinations, the traits of individual offspring are a functions of statistical probability, or the chance of outcome. When you flip a coin there is about an even chance of it coming up either heads or tails. In other words the chance of the coin being heads is roughly one in two. Of course if you flip a coin only twice, you may get tails both times. Statistical probability predicts likely outcome, which will manifest itself over a large sample. If you flip a coin 1000 times, you are most likely to get 500 heads and 500 tails. So when you calculate the likely outcome of genetic combinations, you may find that 1 out of every 10 children is likely to have type B blood. This doesn’t mean that if a set of parents has 10 kids, 1 will definitely type B blood, but only that it is likely to be the case. Humankind has been playing with the genetic code of organisms for thousands of years. Animal husbandry: the mating of domesticated animals to produce offspring with desirable characteristics Gregor Mendel (1822-1884) Agriculture: the merging of organisms to produce crops with desirable characteristics. Mendel is credited with making statistical sense of offspring. By pollinating pea plants with distinct traits with pollens from plants with different traits, Mendel was able to sort out the likelihood of a trait to appear in subsequent generations. Mendel’s work took advantage of several attributes of pea plants. •Ease of pollination •Rapid germination and growth •Distinctive characteristics over generations. He found that many traits were either on or off; purple vs. white flowers, tall vs. short, round seed vs. rumpled. Mendel found traits expressed in parents may not be expressed in the first generation but may be carried over into subsequent generations. Parents of different attributes (like white or purple flowers) could be mated to find which traits were passed along in offspring. What Mendel couldn’t know at the time is that genetic information is stored as chemical sequences in deoxyribonucleic acid (DNA), and that there are two sets of the same DNA region for one trait aspect. However, the statistics remain valid. Alleles are parts of the genetic code (DNA) that defines a trait. There may be more than one allele for a given trait in the cell. (example T for tall, t for short) Genotype refers to all of the alleles present for a trait (example Tt) Phenotype refers to the traits expressed (example: a Tt genotype produced a tall phenotype) Alleles were found by Mendel to be dominant or recessive If a tall plant with genotype TT were mated to a short one of genotype tt, the offspring would have mixed genes, but all would be tall because that allele is dominant Mating these offspring together results three tall plants out of four, one of genotype TT, two of Tt, and one short tt Short (tt) alleles donated to sperm Lets examine the cross in detail. Mendel mated a tall plant (TT) with a short plant (tt). One can make an array of the allele combinations. This makes it easy to see the combinations that result. (The matrix below is known as a Punnett square) Tall (TT) alleles donated to egg F1 T T t Tt Tt t Tt Tt Offspring Four out of every four offspring will have the Tt genotype. Tt is a tall pea plant because the T allele is dominant. If these offspring are mated (don’t worry, incest is an accepted practice among plants), the array shows us their offspring are a mixture. Tall and short (Tt) alleles donated to sperm Tall and short (Tt) alleles donated to egg F2 T t T t T TT Tt TT Tt t Tt tt Tt tt T TT Tt TT Tt t Tt tt Tt tt Four out of every sixteen offspring are short (tt). The remaining twelve are tall, but only four are the genotype TT. Flys need love too… Meet Drosophila melanogaster, or more commonly known as the fruit fly. These arthropods are favorite subjects for introductory genetics labs because they breed quickly and have easily distinguishable traits. We will take some time to look at their reproductive characteristics. Unfortunately, we lack the time to breed them ourselves, which means you won’t have the dubious joy of watching flies twitch in their sleep at 100x magnification. Two traits we can follow: the development of red vs. white eyes and the stumpy (vestigial) vs. full (normal) wings. One further complication: eye color is carried on the gender chromosomes X and Y. The X chromosome can code for either color; the Y chromosome always codes for the recessive white eyes in these critters. Shorthand for alleles XR = red eye Xr = white eye Y = male chromosome (white eye) N = full (normal) wings n = stumpy (vestigial) wings Females have XX, males have XY (like humans) Types Wild = Red eyes and normal wings Mutant = white eyes and stumpy wings Wild Type Drosophila melanogaster F XRXR = red eye NN = normal wings XRY = red eye NN = normal wings M Mutant F M XrXr = white eye nn = vestigial wings XrY = white eye nn = vestigial wings Cross A Wild Type Female x Mutant Male XR XR NN x Xr Y nn What are the probable traits for the first and second generation offspring? F1 Xrn Yn XRN XRN Key XR = red eye Xr = white eye N = normal wings n = vestigial wings Cross A Wild Type Female x Mutant Male XR XR NN x Xr Y nn F1 XRN XRN Xrn XRXrNn XRXrNn Yn XRYNn XRYNn 1/2 Females: red eyed, normal wings 1/2 Males: red eyed, normal winged F2 XRN XRn YN Yn XRN XRn XrN Xrn F2 XRN XRn XRN XRXRNN XRXRNn XRXrNN XRXrNn XRn XRXRNn XRXRnn XRXrNn XRXrnn YN XRYNN XRYNn XrYNN XrYNn Yn XRYNn XrYNn XrYnn XRYnn XrN Xrn 6/16 Females: red eyed, normal wings 3/16 Males: red eyed, normal wings 2/16 Females: red eyed, vestigial wings 1/16 Males: red eyed, vestigial wings 3/16 Males: white eyed, normal wings 1/16 Males: white eyed, vestigial wings Of course the flies and the peas (and the birds and the bees) are interesting in their own right, but they are really useful examples because these apply to all organisms that reproduce sexually (including a large bipedal heterotroph like yourself) Let’s look at a simple human characteristic: blood type There are four types A, B, AB, and O These are phenotypical expressions of the genotypes for blood AA = A AO = A AB = AB OO = O BB = B BO = B This tells us both A and B are dominant alleles (codominant), and O is recessive. Note that if A and B are present on the two different chromosomes, both are expressed as blood type AB.