Unit 10 - Genetics In this unit, we look at how traits are passed on from parent to child.

Genetics Genetics is the study of how traits are passed along from one generation to another and how they interact. Genetics is a relatively new field. People had always known that traits from parents are often passed on to children, but the exact mechanism was not known. We now know that traits are stored in the DNA, which is passed on through the gamete cells of the parents.

Genetics Even before the biochemistry behind inheritance was discovered, the basics were figured out by Gregor Mendel in the 1860s. Working as a monk, he raised pea plants and cross- bred different varieties, carefully observing how traits carried over from generation to generation. He noticed that some plants were true-breeding and would always produce offspring with traits resembling themselves. Other plants would often produce offspring with traits not seen in the parents.

Genetics When doing his experiments he referred to the original parents as the P generation. The first offspring were a combination of the parents’ traits and were thus considered hybrids. They are called the first filial (F 1 ) generation. In turn, the next generation (the grandchildren of the P generation) are the second filial (F 2 ) generation.

Genetics Mendel’s findings: Traits can come in multiple forms called alleles. Every organism has two genes for a trait, it may have two identical alleles, or two different ones. The two gametes (sperm/egg) that form an organism each contribute one of the genes. When an organism has differing alleles, often only one is expressed. This allele is dominant, while the unexpressed allele is recessive.

Genetics In order to be specific about organisms, we specifically refer to their traits in two ways: The phenotype of an organism is it’s observed traits. (ex: purple flowers) These observed traits can however come from multiple gene configurations. This organism may have two purple-color alleles, or may only have one that is dominant, hiding another allele.

Genetics The actual genetic makeup of an organism is it’s genotype. If an organism had two identical alleles for a trait, it is homozygous. If it has two differing alleles, it is heterozygous. Dominant traits are written with a capital letter, while the recessive trait is written in lowercase. (ex: a heterozygous organism could be written as “Pp”)

Genetics The actual genetic makeup of an organism is it’s genotype. If an organism had two identical alleles for a trait, it is homozygous. If it has two differing alleles, it is heterozygous. Dominant traits are written with a capital letter, while the recessive trait is written in lowercase. (ex: a heterozygous organism could be written as “Pp”)

Genetics When we breed two organisms and observe the inheritance of one trait, we are observing a monohybrid cross. Sometimes we want to observe two separate traits. In this case we are looking at what alleles exist at multiple locations ( or loci ) on the DNA. In the case of comparing two genes, we observe a dihybrid cross.

Punnett Square To predict the outcomes of certain crosses, we set up a Punnett Square. In this chart, the parents are set on the top and side. In this example, we have a situation where pea pod color has a dominant yellow allele (Y) and a recessive green allele (y).

Punnett Square The parent on the top is homozygous recessive (yy) and thus has green pea pods. The other parent is heterozygous (Yy) and thus shows the dominant yellow color.

Punnett Square To complete the square, we combine each of one parent’s alleles with each of the other parents. The green parent (yy)can only pass on a green (y) allele as that is all it has. However, the other parent (Yy) may pass on either a yellow (Y) or a green (y) allele.

Punnett Square The results show that two of the possible four children will be Yy (heterozygous) while the other two will be yy (homozygous recessive). Depending on the parents, the offspring may show a ratio, give multiple different types of offspring or even only show offspring of one variety.

Punnett Square Dihybrid crosses require a bit more preparation before we set up the table. For each of the two traits, each parent can give either of its alleles for either trait. The two are independent. That means that with a parent of the genotype AaBb, the A and B do not have to be passed on together – the A has an equal chance to be passed along with either the B or b.

Punnett Square A parent with AaBb genotype gives off the following possible gametes: AB Ab aB ab When building a dihybrid cross, you must generate all the possible combinations for both parents first.

Punnett Square In this cross, both parents are AaBb genotypes. After doing the cross, we find some offspring resemble the parents (either genotypically or phenotypically), but many other new combinations arise.

Punnett Square It is very beneficial that traits to all inherit independently – it leads to new combinations and thus increases diversity and the chances of offspring surviving.

Incomplete Dominance In all the examples we have done so far, one trait dominates the other, and thus heterozygous organisms only show the dominant trait. In incomplete dominance, both traits show through in heterozygotes. For example, if an organism had both a blue and a red allele, it would show up as purple. When writing these alleles, we write both in capitals as they both show through.

Incomplete Dominance W – white petals R- red petals Here is a cross of a red (RR) and white (WW) plant. All offspring are heterozygous and phenotypically pink.

Co-Dominance Co-dominance is similar to incomplete dominance, but instead of both traits blending (like blue + red=purple), in co-dominance, both are expressed without mixing. Blood types in people work through co-dominance. Type A and B are dominant over O, but in people who are AB, both A and B are expressed at the same time, leading to the person having both traits and not some mixture.

Blood Types Blood types work through three alleles rather than the usual two we’ve done so far. A person can have a combination of A, B or O alleles. A and B each code for differing carbohydrate chains to be put onto blood cells for recognition. The O allele does nothing. A person with AB blood has both an A and B allele and so their cells have both types of carbohydrate on them.

Blood Types A person with A-type blood may have AA or AO genotype. Similarly a B-type person may be BB or BO. As the O allele codes for nothing, it is effectively recessive. Type-O people are OO genotype only. The body will attack blood cells that have carbohydrates on it that it does not normally see. Thus if an Type-A person gets Type-B blood, it causes problems.

Blood Types Everyone can take Type-O blood because it has no carbohydrate markers and thus cannot cause a reaction. However, Type-O people can only recieve Type-O blood because both A and B carbohydrates would cause problems to them. Type-AB people can receive all types of blood because they are used to both A and B carbohydrates.

Blood Types Blood type can be used to rule out certain people as possible parents in some cases. For instance, a Type-AB man could never have a Type-O child, as he could only pass on an A or a B allele, and not one of the two O alleles the child has. Of course, blood types cannot prove paternity. More in-depth DNA tests are done to test for parentage.

Sex Chromosomes In humans and many organisms, sex is determined by a pair of chromosomes. We all have the same 22 chromosomes, the autosomes, but which of the possible sex chromosomes (the 23 rd pair), determines sex. XX – female XY – male Note that YY is not possible. This is because a mother can only give a child an X, so they can never get two. As well, the X chromosome contains general-purpose genes and not just female-specific ones, so everyone needs at least one X.

Sex Chromosomes In humans and many organisms, sex is determined by a pair of chromosomes. We all have the same 22 chromosomes, the autosomes, but which of the possible sex chromosomes (the 23 rd pair), determines sex. XX – female XY – male Note that YY is not possible. This is because a mother can only give a child an X, so they can never get two. As well, the X chromosome contains general-purpose genes and not just female-specific ones, so everyone needs at least one X.

Sex Chromosomes The Y chromosome contains a number of genes that are important for developing an organism into a male. Without these genes, an organism will develop into a female. Both the X and Y do hold many general-purpose genes. Unlike in the autosomal genes, these get passed on in different ways and ratios. This is due to women having two X and men having only one X and one Y.

Sex Chromosomes Sex-linked traits are passed on in limited ways compared to autosomal traits. This is because a woman passes on either of her X chromosomes and a man can only pass on an X or a Y. Y-linked traits are passed from father to son 100% of the time. However they cannot be passed from grandfather to mother to son, as that woman did not have a Y Chromosome to carry the gene. Similarly, a father will pass on any X-linked traits to all of his daughters.

Sex Chromosomes Mothers, however, contribute only one of their X chromosomes to a child and thus they may or may not pass on sex-linked traits. Some sex-linked traits:  Red-green colorblindness  Male Pattern Baldness  Hemophilia  Duchene Muscular Dystrophy