• Experimental tool: garden pea
• Outcome of genetic cross is independent of whether the genetic trait comes from the male or female parent
• Reciprocal genetic crosses produce the same results
• Many human traits follow this pattern of inheritance

• Gene : inherited trait
• Plants with different forms of a trait, such as yellow vs. green seeds
( alleles) were genetically crossed
• Mendel counted the number of offspring with each trait (F1), (e.g.: green seeds)
• He crossed F1 plants among themselves and counted F2 offspring

• Genetic cross between parents that
“breed true” for a pair of traits, round seeds vs. wrinkled seeds , produces offspring with round seeds only (F1)
• Round seeds are dominant
• Each parent has two identical copies of the genetic information specifying the trait
(homozygous) and contributes one in each cross (P1)

• Round seed parent
“AA” = genotype
• Wrinkled seed parent
• Round seed parent contributes “A” gamete to offspring
• Wrinkled seed parent contributes “a” gamete to offspring

• Offspring genotype = A + a = Aa heterozygous
• All offspring produce round seeds although they are genetic composites of
“Aa” because “A” (round) is dominant to
“a” (wrinkled)

• F1 genotype =“Aa”= monohybrid
• “Aa” parent produces either “A” or “a” gametes in equal proportion
Law of Segregation
( simple consequence of two chromosomes)

• Genetic cross : Aa X Aa produces A and a gametes from each parent
• Punnett square shows four possible outcomes = AA Aa, aA, and aa
• Three combinations = AA, Aa, and aA produce plants with round seeds and display a round phenotype
• Fourth combination = aa displays wrinkled phenotype = recessive

Chart Title: Monohybrid genetic Cross
Parents: Aa X Aa gametes: A or a each parent produces A and a gametes and contributes one gamete at fertilization
1/4
AA round seeds dominant
1/2
Aa round seeds dominant
1/4 aa wrinkled seeds recessive

• Genotypic ratios differ from phenotypic ratios since dominant phenotype consists of AA” and “Aa”
• F2 results of monohybrid cross show 3:1 round:wrinkled phenotypic ratio
• Genotypic ratios of monohybrid cross are
1:2:1 = 1/4 AA + 1/2 Aa + 1/4 aa

• Testcross analysis allows geneticist to determine whether observed dominant phenotype is associated with a homozygous “AA” or heterozygous “Aa” genotype
• Genetic cross is performed using a recessive testcross parent = “aa”

• AA + aa = Aa ; dominants only parent homozygous
• Aa + aa = 1/2 Aa + 1/2 aa produces 1/2 dominant, 1/2 recessive parent heterozygous

• two different phenotypic traits , such as seed color
(yellow vs. green) and seed shape (round vs. wrinkled)
• Analysis of all combinations: (3:1 round : wrinkled and 3:1 yellow : green) produces
9:3:3:1 phenotypic ratio (round/yellow : round/green : wrinkled/yellow : wrinkled/green

• Combinations of individual elements within dihybrid pair generate genotypic ratios for dihybrid cross
• True for any number of unlinked genes
• Also a consequence of distinct chromosomes

• WwGg gametes = WG
+ wG +Wg + wg = 1:1:1:1 ratio;
• double recessive gametes = wg
• Offspring = WwGg + wwGg + Wwgg + wwgg = 1:1:1:1 ratio
• Testcross shows that parent is heterozygous for both traits (dihybrid)

• Trihybrid cross = three pairs of elements that assort independently, such as
WwGgPp
• For any pair phenotypic ratio = 3:1
• For two pairs ratio = 9:3:3:1
• Trihybrid: 27:9:9:9:3:3:3:1

• Addition Rule : The probability of obtaining one or the other of two mutually exclusive events is the sum of their individual probabilities
• Multiplication Rule : The probability of two independent events occurring simultaneously equals the product of their individual probabilities

• Dihybrid crosses also follow sum rule and product rule to determine outcome probabilities
• Phenotypic outcome = 9:3:3:1
• Genotypic outcome = 1:2:1:2:4:2:1:2:1

• In humans, pedigree analysis is used to determine individual genotypes and to predict the mode of transmission of single gene traits
• To construct a pedigree, the pattern of transmission of a phenotypic trait among individuals in a family is used to determine whether the mode of inheritance is dominant or recessive
• Pedigree analysis is used to study single gene disorders , such as Huntington’s
Disease, a progressive neurodegenerative disorder

• Dominant phenotypic traits usually appear in every generation of a pedigree
• About 1/2 the offspring of an affected individual are affected
• The trait appears in both sexes if the gene is not on the X chromosome

Transmission Probabilities for Dominant Single Gene Traits most common cross
Aa X aa
Aa = affected aa = nonaffected
Aa affected heterozygote prob = 1/2
A = defective genetic element aa nonaffected recessive prob = 1/2 a = nonaffected genetic element

• Pedigree analysis can used to distinguish dominant vs. recessive modes of inheritance for traits determined by single genes
• Analysis of patterns of transmission of recessive genes is used to identify carriers of recessive traits which cannot be determined by direct phenotypic analysis
• Recessive traits occur in individuals whose parents are phenotypically dominant

• Two phenotypically dominant people who produce a child with a recessive genetic disorder:
1/4 probability that any of their children will be affected and 1/2 that they will be carriers

Inheritance of Recessive Single Gene Disorders most common cross
Aa X Aa
A = nonaffected gene a = affected gene
AA prob = 1/4 nonaffected
Aa prob =1/2 carrier aa prob = 1/4 affected

• Heterozygote phenotype is intermediate between dominant and recessive phenotypes (snapdragons)
• F1 of cross between dominant
(red) and recessive (ivory) plants shows intermediate phenotype (pink)
• F2 products show identical phenotypic and genotypic ratios

• For some traits more than two alleles exist in the human population
• ABO blood groups are specified by three alleles which specify four blood types
• ABO blood group inheritance also illustrates principle of co-dominance in which both alleles contribute to the phenotype in the heterozygote
• Antibodies are proteins which bind to stimulating molecules = antigens

• IA and IB are dominant to IO, genotype AIO = type A; IBIO = type B
• IA and IB are co-dominant; each allele specifies antigen: genotype
IAIB = type AB
• IO = is recessive genotype IOIO

• Many recessive genes code for enzymes which carry out specific steps in biochemical pathways
• Mutations which alter the structure of genes block enzyme production if both copies of the gene are defective
• Disorders were termed “inborn errors of metabolism” by Garrod

• Recessive genes often contain mutations which block the formation of gene product (ww)
• Heterozygotes which contain one recessive gene copy (Ww) may produce only 1/2 the amount of protein specified by the homozygous dominant (WW) which contains two functional copies of the gene

• Heterozygotes (Ww) may still produce sufficient gene product to display dominant phenotype = round seed; genotype = carrier
• For some genes reduction of gene product by 1/2 in the heterozygote may be physiologically significant, especially for structural proteins = dominant disorders

• Variable expressivity refers to genes that are expressed to different degrees in different individuals, e.g.: severity of an inherited disease
• Incomplete penetrance means that the phenotype predicted from a specific genotype is not always expressed, e.g.: individual inherits mutant gene but shows no effect

• Epistasis alters Mendelian
9:3:3:1 phenotypic ratios in dihybrid inheritance
• In epistasis, two sets of genetic elements interact to produce a single phenotype, which modifies the observed phenotypic ratios
• Mendelian pattern of inheritance

• Complementation tests are used to determine if different phenotypes result from variations in one gene
• Homozygous recessive genotypes which are genetically crossed can only produce a dominant phenotype if the recessive genetic elements are located on different genes

• A mutant screen is an experiment which generates mutations which affect specific phenotypes
• Multiple alleles refer to the various forms of a gene
• Wildtype refers to the phenotype for a specific trait most commonly observed

• The complementation test groups mutants into allelic classes called complementation groups
• Lack of complementation = two mutants are alleles of the same gene
• Principle of Complementation : two recessive allelic mutations produce mutant phenotype; two non-allelic recessive mutations show no effect