Chapter 4: Mendel and the Gene Idea. The Experiments of Gregor Mendel 2. Beyond Mendelian Genetics 3. Human Genetics
. The Experiments of Gregor Mendel Chapter Reading pp. 268-276
TECHNIQUE Parental generation (P) RESULTS First filial generation offspring (F ) 2 Stamens Carpel 3 4 5 Gregor Mendel Deduced the basics of inheritance in the 9 th century by analyzing pea plant crosses: examined several characters exhibiting dominant & recessive traits or phenotypes
Mendel s EXPERIMENT Crosses P Generation (true-breeding parents) Purple flowers White flowers ) Cross plants that breed true for character of interest F Generation (hybrids) All plants had purple flowers 2) Cross the resulting hybrids (a monohybrid cross) F 2 Generation 705 purple-flowered plants 224 white-flowered plants
Results for Various Characters For each character Mendel saw the same basic pattern: a 3: ratio of dominant to recessive traits following a monohybrid cross
Phenotype vs Genotype Phenotype Genotype Purple PP (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp (homozygous) Ratio 3: Ratio :2:
P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Mendel s Law of Segregation Gametes: F Generation Appearance: Genetic makeup: Gametes: F 2 Generation Eggs P Purple flowers Pp / 2 P / 2 P P PP Sperm p Pp p p Each gamete ends up with only one of the two copies of each gene in the parent. e.g., either the purple flower allele or the white flower allele 50% of gametes contain one copy, 50% contain the other p Pp 3 pp This is due to the segregation of chromosomes during meiosis.
Test Crosses TECHNIQUE P? x pp Test crosses are used to reveal an unknown genotype: cross the unknown with a homozygous recessive individual Predictions Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP If Pp or Sperm Sperm p p p p if all offspring display dominant phenotype: homozygous dominant (PP) P Eggs P Pp Pp Pp Pp Eggs P p Pp pp Pp pp RESULTS if ~½ each of recessive and dominant phenotypes: heterozygous (Pp) All offspring purple or / 2 offspring purple and / 2 offspring white
Genetic Alleles & Genetic Loci Genes can have different versions known as alleles. e.g., purple flower vs white flower alleles Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers All genes have a specific chromosomal location referred to as its locus. homologous chromosomes have the same genetic loci
Important Genetic Terms Character physical characteristic (e.g., eye color) Phenotype individual s observable traits (e.g., brown eyes) Allele different forms or versions of the same gene Genotype the alleles an individual has for a given gene (e.g., AA or Aa or aa)
Homozygous the 2 alleles for a gene are identical (AA or aa) Heterozygous the 2 alleles for a gene are different (Aa) Dominant allele (designated by capital letter: A) copy of the allele determines phenotype (A ) Recessive allele (designated by lower-case letter: a) affects phenotype only when homozygous (aa) Homozygous dominant = AA Homozygous recessive = aa Heterozygous = Aa
Genetics & Probability All genetic inheritance is based on the probability of inheriting specific genetic alleles from each parent. Rr Segregation of alleles into eggs / 2 Eggs R / 2 R R R Sperm / 2 R Rr Segregation of alleles into sperm r r / 2 r r R r r
The Nature of Probability The probability of multiple events happening simultaneously is the product of the probabilities of each single event. The probability of one event OR another is the sum of the probabilities of each event.
Multiple Gene Inheritance EXPERIMENT P Generation F Generation Predictions Predicted offspring of F 2 generation Eggs / 2 / 2 YR yr YYRR Gametes YR Hypothesis of dependent assortment Sperm / YR 2 / 2 yr 3 YYRR YyRr YyRr yyrr or Phenotypic ratio 3: yr YyRr Eggs yyrr Hypothesis of independent assortment YR Yr yr yr Sperm / YR 4 / 4 Yr yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr yr YyRr Yyrr yyrr yyrr In a dihybrid cross, the probability of each combined phenotype is the product of each individual phenotype. Expected Ratios: 9 Y R 3 Y rr RESULTS 35 08 0 32 9 / 6 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3: 3 yyr yyrr
Mendel s Law of Independent Assortment Each pair of genetic alleles segregates independently of all other pairs of genetic alleles during gamete formation (meiosis). the distribution of the alleles of a given gene into gametes has no connection to the distribution of alleles for other genes applies only to genes on different chromosomes
2. Beyond Mendelian Genetics Chapter Reading pp. 276-28
P Generation Incomplete Red C R C R White C W C W Dominance F Generation Gametes C R C W Pink C R C W Dominant allele is expressed to lesser degree when only copy is present (heterozygous): C R Gametes / 2 / 2 C W F 2 Generation Eggs / 2 C R C R Sperm / 2 / 2 C W C R C R C R C W heterozygotes have an intermediate phenotype (e.g., pink vs red or white) / 2 C W C R C W C W C W
Codominance Two different alleles when paired together are expressed equally (e.g., ABO blood type) Allele I A Carbohydrate I B B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype A Red blood cell appearance Phenotype (blood group) I A I A or I A i A A & B alleles are codominant (both alleles expressed) I B I B or I B i I A I B B AB O allele is recessive ii (b) Blood group genotypes and phenotypes O
Eggs BC bc Sperm BbCc BbCc BC bc Bc bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc Epistasis The expression of a genetic allele from one locus alters the expression of alleles at another locus. Bc bc BBCc BbCc BBcc BbCc bbcc Bbcc 9 : 3 : 4 Bbcc bbcc in this example the homozygous recessive genotype for one gene overrides the expression of the other gene
Polygenic Inheritance Sperm AaBbCc AaBbCc / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Many phenotypes are due to the effects of multiple genes (such as skin color). Eggs / 8 / 8 / 8 / 8 / 8 in this example, 3 different genes influence one phenotype resulting in 64 different variants Phenotypes: / 8 / 8 / 8 / 64 6 / 64 5 / 64 20 / 64 5 / 64 6 / 64 / 64 Number of dark-skin alleles: 0 2 3 4 5 6
3. Human Genetics Chapter Reading pp. 282-285
Key Male Female st generation (grandparents) 2nd generation (parents, aunts, and uncles) Affected male Affected female Ww Ww ww ww ww Ww ww Mating Offspring, in birth order (first-born on left) Ww Ww ww Pedigrees Diagrams illustrating family history that allow inheritance patterns to be deduced. 3rd generation (two sisters) Widow s peak WW or Ww ww (a) Is a widow s peak a dominant or recessive trait? No widow s peak We can t do experimental crosses on humans, we can only analyze existing family pedigrees.
A Recessive Human Trait st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?
Albinism is also Recessive Parents Normal Aa Normal Aa Sperm A a Eggs A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino
Achondroplasia is Dominant Parents Dwarf Normal Dd dd Sperm D d Eggs d Dd Dwarf dd Normal d Dd Dwarf dd Normal
Key Terms for Chapter 4 trait, phenotype, genotype, allele, locus homozygous, heterozygous dominant, recessive, codominant true-breeding, monohybrid, dihybrid test cross, Punnet square epistasis polygenic inheritance pedigree Relevant Chapter Questions -4