Chapter 9 1 Mendel s decision to work with peas was good choice because the petals on the pea flower almost completely enclose the reproductive organs the stamens and carpel Also and perhaps the most important advantage of the pea plant as an experimental model was that Mendel could strictly control matings Consequently pea plants usually are able to self fertilization in nature Mendel worked with his plants until he was sure he has true bleeding varieties varieties for which self fertilization produced offspring all identical a purple flowered variety that when self fertilized produced off spring plants that all had purple flowers The offspring of two different varieties are called hybrids The true bleeding parents are called the P generation and their hybrid offspring are called the F1 generation When the F1 plants self fertilize or fertilize each other their offspring are the F2 generation A heritable feature that varies among individuals such as a flower color is called a character Each variant for a character such as purple or white flowers is called a trait 2 An organism that has two identical alleles for a gene is said to be homozygous for that gene An organism that has two different alleles for a gene is said to be heterozygous for that age If the two alleles of an inherited pair differ then one determines the organism s appearance and is called the dominant allele the other has no noticeable effect on the organism s appearance and is called the recessive allele Because an organism s appearance does not always reveal its genetic composition geneticists distinguish between an organism s physical trait called its phenotype and its genetic makeup its genotype A cross between a true bleeding pea plant with purple flowers and true bleeding pea plant with white flowers This is an example of monohybrid cross because it follows just one character flower color A dihybrid cross is a mating of parental varieties differing in two characters Alternative versions of a gene are called alleles Locus a specific location of a gene along the chromosome A Punnett square repeats the cross in a way that highlights the four possible combination of gametes and the resulting four possible offspring in the F2 generation How can two plants with different genotypes for a particular inherited character be identical in phenotype One could be homozygous for the dominant allele and the other heterozygous Mendel s law of segregation states that a sperm or egg carries only one allele for each inherited character because allele pairs separate segregate from each other during the production of gametes A RR x rr 0 B Rr x Rr or 25 C Rr x rr or 50 Chapter 9 3 Mendel needed to explain why the F2 offspring has new nonparental combinations of trait and had a 9 3 3 1 phenotypic ratio He suggested that the inheritance of one character has no effect on the inheritance of another suggest that the dihybrid cross is the equivalent to two monohybrid crosses and called this the law of independent assortment 4 A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual Geneticists use the testcross to determine unknown genotypes A testcross can show whether the unknown genotype includes a recessive allele 5 A pedigree shows the inheritance of a trait in a family through multiple generations demonstrates dominant or recessive inheritance and can also be used to deduce genotypes of family members Most people who recessive disorders are born to normal parents who are both heterozygotes carriers of recessive allele for the disorder but are phenotypically normal 6 In a complete dominance the dominant allele has the same phenotypic effect whether present in one or two copies But for some characters the appearance of F1 hybrids falls between the phenotypes of the two parental varieties an effect called incomplete dominance The A and B alleles are both expressed in heterozygous individuals making both alleles co dominant Pleiotropy occurs when one gene influences many characters 7 Skin color is affected by exposure to sunlight heart disease and cancer are influenced by genes and the environment Identical twins show that a person s traits are results of genetic and the environment 8 Linked genes which are located close together on the same chromosomes and tend to be inherited together Linked genes tend to be inherited together and not sort independently because they are located close together on the same chromosome 9 Among humans individuals with one X chromosome and one Y chromosome are males XX individuals are females The X Y system is only one of several sex determining systems 10 Males only have one copy of the X chromosome which means that only one recessive allele is needed to cause a sex linked disorder whereas with females since they have two copies of X both chromosomes must contain the recessive allele for it to cause the disease 11 The Y chromosome can be particularly useful for tracing our evolutionary past because barring mutation the human Y chromosome passes essentially intact from father to son