UNIT 4:GENETICS ANDINHERITANCECampbell & Reece:Chapters 14 and 15
1. WHAT IS GENETICS• Genetics: The study of heredity.• Heredity is the relations betweensuccessive generations.• Why do children look a little bit like theirparents but also different?• What is responsible for thesesimilarities and differences?
2. MENDEL’S GENETICS• Gregory Mendel is the father ofGenetics.• Mendel discovered the basic principlesof heredity by breeding garden peas incarefully planned experiments.• Advantages of pea plants for geneticstudy: Cross-pollination (fertilizationbetween different plants) can be achieved bydusting one plant with pollen from another.
• Cross-pollination (fertilization betweendifferent plants) can be achieved by dustingone plant with pollen from another.
• He also used varieties that were true-breeding(organisms with only one variety of a type e.g.red flowers can only produce red flowers)• In a typical experiment, Mendel mated twocontrasting, true-breeding varieties, a processcalled hybridization• The true-breeding parents are the Pgeneration.• The hybrid offspring of the P generation arecalled the F1 generation• When F1 individuals self-pollinate, the F2generation is produced
• When Mendel crossed contrasting, true-breeding white and purple flowered peaplants, all of the F1 hybrids were purple
• When Mendel crossed the F1hybrids, many of the F2 plants had purpleflowers, but some had white• Mendel discovered a ratio of about threeto one, purple to white flowers, in the F2generation.
• Mendel reasoned that only the purple flowerfactor was affecting flower color in the F1hybrids.• Mendel called the purple flower color adominant trait and the white flower color arecessive trait• What Mendel called a “heritable factor” iswhat we now call a gene• He did 7 other crosses using different traitsand found the same phenomenon.
• Mendel noted that the gene for flower color forexample exists in two versions, one for purpleflowers and the other for white flowers• These alternative versions of a gene are nowcalled alleles• Each gene is found at a specific locus (position) ona specific chromosome.
• The two alleles at a locus on a homologouschromosome pair may be identical, as in thetrue-breeding plants – they are then said tobe homozygous for that trait/gene.• Alternatively, the two alleles at a locus maydiffer – they are said to be heterozygous forthat gene/trait.• If the two alleles at a locus differ, then one(the dominant allele) determines theorganism’s appearance (we refer to it as itsphenotype), and the other (the recessiveallele) has no noticeable effect on
• Mendel then formulated the law ofsegregation, states that the two alleles for aheritable character separate (segregate)during gamete formation and end up indifferent gametes• Thus, an egg or a sperm gets only one of thetwo alleles that are present in the somaticcells of an organism.
• An organism traits are indicated via itsgenotype and phenotype. Genotype: The genetic composition ofthe gene, indicated by letters e.g.GG, Gg, gg. (A capital letter representsa dominant allele, and a lowercaseletter represents a recessive allele) Phenotype: The external appearance ofthe gene e.g. Brown hair, white hair.
3. GENETIC CROSSES• HOW CAN WE NOW MORE OF LESSDETERMINE WHAT WILL BE THEOUTCOME IF 2 ORGANISMS HAVE ABABY?
TWO TYPES OFGENETIC CROSSES• MONOHYBRID CROSSES: A crossbetween 2 organisms where we onlylook an one pair of contrasting traits.• DIHYBRID CROSS: A cross between 2organisms where we look at two pairsof contrasting traits at the same time.
MONOHYBRID CROSS -EXAMPLE• Determine the outcome/ F1 generationof a cross between a homozygous tallplant and a homozygous short plant.Tall plants are dominant over shortplants.
STEPS TO SOLVE A CROSS PROBLEM1. What trait are we looking at?2. Choose a letter to represent the trait.3. See if you can identify which trait is dominant –allocate the capital letter to that trait.4. Identify the recessive trait and allocate a lower caseletter to that trait.5. Determine the genotypes of the parents. –Homozygous dominant – Two capital letters e.g. GGHomozygous recessive – Two lower case letter. E.g.ggHeterozygous – One capital letter and one lowercase letter e.g. Gg1. Start with cross
SOLUTION1. Trait – Size of plant.2. Letter chosen to represent size of plant = T/t3. Tall plants are dominant. (Given in problem) –Given the – “T” (capital T)4. Short plants are recessive – given the “t”(lower case t)5. One parent is homozygous tall – TTother parent is homozygous short - tt
CROSS SHOWN AS A GENETIC DIAGRAMWhy?Tall is dominant over short plants – Babies have bothalleles: tall and short
CROSS BETWEEN F1 GENERATIONINDIVIDUALS (INTERBREED F1 GENERATION)
MONOHYBRID CROSS –EXAMPLE 2A heterozygous blue eyed rabbit is crossedwith a rabbit with pink eyes. What is thepossibility of the babies being born withpink eyes?
SOLUTION1. Trait: eye colour of rabbit.2. Letter used: E/e3. Dominant trait: Blue eyes (Why? The first rabbit isheterozygous – both alleles – but blue is beingexpressed in rabbit eyes.) = E4. Recessive trait: pink eyes = e5. Rabbit one – heterozygous: EeRabbit two – homozygous: ee (why?)The only way that a rabbit can have pink eyesexpressed externally is if both alleles code for pinkeyes.
EXAMPLE OF A DIHYBRID CROSSDetermine the F2 generation of a crossbetween yellow round seeded peas andwrinkled green seeded peas. Yellow andround seeds are dominant.
• Using a dihybrid cross, Mendel developedthe law of independent assortment• The law of independent assortment statesthat each pair of alleles segregatesindependently of each other pair of allelesduring gamete formation.• Strictly speaking, this law applies only togenes on different, nonhomologouschromosomes• Genes located near each other on thesame chromosome tend to be inheritedtogether.
4. DEGREES OF DOMINANCE•Complete dominance One allelesuppresses the expression of the otherallele.• Incomplete dominance: phenotype of F1hybrids is somewhere between thephenotypes of the 2 parental varieties –neither allele completely dominant(White x Red = Pink)• Codominance, 2 dominant alleles affectthe phenotype inseparate, distinguishable ways. (Redand white flowers = White and red
5. MULTIPLE ALLELES Most genes exist in populations in more than twoallelic forms. For example, the four phenotypes of the ABOblood group in humans are determined by threealleles for the enzyme (I) that attaches A or Bcarbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the Acarbohydrate, whereas the enzyme encoded bythe IB allele adds the B carbohydrate; theenzyme encoded by the i allele adds neither.
6. PLEIOTROPY Most genes have multiple phenotypic effects, aproperty called pleiotropy For example, pleiotropic alleles are responsiblefor the multiple symptoms of certain hereditarydiseases, such as cystic fibrosis and sickle-celldisease
7. Polygenic Inheritance Polygenic inheritance is an additive effect of twoor more genes on a single phenotype Skin color in humans is an example of polygenicinheritance.
8. DETERMINING THE SEX OF A BABY In humans and other mammals, there aretwo varieties of sex chromosomes: a largerX chromosome and a smaller Ychromosome Only the ends of the Y chromosome haveregions that are homologous with the Xchromosome The SRY gene on the Y chromosome codesfor the development of testes.
9. Inheritance of Sex-Linked Genes The sex chromosomes have genes for manycharacters unrelated to sex A gene located on either sex chromosome iscalled a sex-linked gene In humans, sex-linked refers to a gene on thelarger X chromosome. For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele. Sex-linked recessive disorders are much morecommon in males than in females.
Some disorders caused by recessive alleles onthe X chromosome in humans:• Color blindness• Duchenne muscular dystrophy• Hemophilia