Presentation on theme: "23.1 Mendel’s Laws Gregor Mendel Augustinian Monk"— Presentation transcript:
1 23.1 Mendel’s Laws Gregor Mendel Augustinian Monk Around 1857, began breeding garden peas to study inheritance.Performed crosses between true breeding lines of garden peas that differed in a single trait.
2 Pea plants have several advantages for genetics: Pea plants are available in many varieties with distinct heritable features (characters) with different variants (traits).CharacterTraitsSeed shapeRoundWrinkledSeed colorYellowGreenPod shapeInflatedConstructedPod colorGreenYellow
3 Mendel could control which plants mated with which. Each pea plant has male (stamens) and female (carpal) sexual organs.In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm.However, Mendel could also move pollen from one plant to another to cross-pollinate plants.
5 Cross-Pollination 1. Remove anthers from one plant. 2. Collect pollen from a different plant.3. Transfer pollento a stigma of theindividual whoseanthers have beenremoved.
6 Mendel’s ExperimentIn a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.Hypotheses:All offspring will be a blend of the two colors (lavender)All offspring will be some of each colorAll offspring will be one color or the other
7 Mendel then allowed the F1 hybrids to self-pollinate to produce an F2 generation. When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants.The white trait, absent in the F1, reappeared in the F2.Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross (a ratio of three purple to one white flowering plant in the F2 offspring).
8 Mendel’s ConclusionsMendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color.Purple flower is a dominant trait and white flower is a recessive trait.Mendel’s quantitative analysis of F2 plants revealed the two fundamental principles of heredity:law of segregationlaw of independent assortment.
9 Law of SegregationFour related ideas:1. Different factors or alternative versions of genes (alleles) account for variations in inherited characters.Different alleles vary somewhat in the sequence of nucleotides at the specific locus (location) on paired chromosomes.The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus.
10 2. For each character, an organism inherits two alleles, one on each homologous chromosome from each parent.Each diploid organism has a pair of homologous chromosomes and therefore two copies of each locus.A diploid organism inherits one set of chromosomes from each parent.These homologous loci may be identical, as in the true-breeding plants of the P generation.Alternatively, the two alleles may differIn the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other.
11 3. If two alleles differ, then one, the dominant allele, is fully expressed in the the organism’s appearance.The other, the recessive allele, has no noticeable effect on the organism’s appearance.Mendel’s F1 plants had purple flowers because the purple-flower allele is dominant and the white-flower allele is recessive.
12 4. The two alleles for each character segregate (separate) during gamete production when the homologous chromosomes are separated and distributed to different gametes in meiosis.If an organism has identical alleles for a particular character, then that allele exists as a single copy in all gametes.If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other.The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.
13 Law of Segregation - Summary Each individual has alleles for each traitThe alleles segregate (separate) during the formation of gametesEach gamete contains only one allele from each pair of allelesFertilization gives each new individual two alleles for each trait
15 Inheritance of a Single Trait Phenotype: physical appearance of the individual with regard to a traitGenotype: alleles responsible for a given traitTwo alleles for a traitA capital letter symbolizes a dominant allele (W)A lower-case letter symbolizes a recessive allele (w)Dominant refers to the allele that will mask the expressionof the alternate (recessive) allele
17 Single Trait Gamete Formation During meiosis, homologous chromosomes separate so there is only 1 member of each pair in a gameteThere is one allele for each trait, such as hairline, in each gameteExample: if one parent’s genotype is Ww, then some gametes from this individual will contain a W and others a w
18 One-Trait CrossA homozygous man with a widow’s peak X a woman with a straight hairline
20 One-Trait Crosses and Probability The chance of 2 or more independent events occurring together is the product of their chance of occurring separatelyIn the cross Ww X Ww, what is the chance of obtaining either a W or a w from a parent?Chance of W = ½ and the chance of w = ½Therefore the probability of having these genotypes is as followsChance of WW= ½ X ½ = ¼Chance of Ww = ½ X ½ = ¼Chance of wW= ½ X ½ = ¼Chance of ww = ½ X ½ = ¼
21 One-Trait Test CrossBreeders of plants and animals may do a test cross to determine the likely genotype of an individual with the dominant phenotypeCross with a recessive individual - the recessive has a known genotype (ww)If there are any offspring produced with the recessive phenotype, then the dominant parent must be heterozygous
23 Inheritance of Two Traits The Law of Independent Assortment:Each pair of factors assorts independently (without regard to how the others separate)All possible combinations of factors can occur in the gametes
26 Two-Trait Crosses (Dihybrid Cross) WwSs (X) WwSsPhenotypic Ratio:9 widow’s peak, short fingers3 widow’s peak, long fingers3 straight hairline, short fingers1 straight hairline, long fingers
27 Two-Trait Crosses and Probability Probability LawsProbability of widow’s peak = ¾Probability of short fingers= ¾Probability of straight hairline= ¼Probability of long fingers= ¼Using the Product RuleProbability of widow’s peak and short fingers =¾ X ¾ = 9/16Probability of widow’s peak and long fingers =¾ X ¼ = 3/16Probability of straight hairline and short fingers =¼ X ¾ = 3/16Probability of straight hairline and long fingers =¼ X ¼ = 1/16
28 Pedigree AnalysisInformation about the presence or absence of a particular phenotypic trait is collected from as many individuals in a family as possible and across as many generations as possible.The distribution of these characters is then mapped on the family tree.
29 Example: If an individual in the third generation lacks a widow’s peak, but both her parents have widow’s peaks, then her parents must be heterozygous for that gene.If some siblings in the second generation lack a widow’ peak and one of the grandparents (first generation) also lacks one, then the other grandparent must be heterozygous and we can determine the genotype of almost all other individuals.
30 Beyond Simple Inheritance Patterns Incomplete DominanceOccurs when the heterozygote shows a distinct intermediate phenotype not seen in the two homozygotesOffspring of a cross between heterozygotes will show three phenotypes: each parent and the heterozygote.The phenotypic and genotypic ratios are identical, 1:2:1.
31 A clear example of incomplete dominance is seen in flower color of snapdragons. A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring.Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring.
33 CodominanceOccurs when alleles are equally expressed in a heterozygoteExample: the M, N, and MN blood groups of humans are due to the presence of two specific molecules on the surface of red blood cells.People of group M (genotype MM) have one type of molecule on their red blood cells, people of group N (genotype NN) have the other type, and people of group MN (genotype MN) have both molecules present.
34 Multiple Allele Inheritance A trait is controlled by multiple alleles, the gene exists in several allelic forms.Each person has only two of the possible alleles.ABO Blood TypesIA = A antigens on red blood cellsIB = B antigens on red blood cellsi = has neither A nor B antigens on red blood cellsBoth IA and IB are dominant over i, IA and IB are codominant
35 ABO Blood Types Phenotype Genotype A IAIA or IAi B IBIB or IBi AB IAIB O iiBoth IA and IB are dominant over i, IA and IB are codominantThe Rh factor is inherited separately from ABO blood types.
37 Sex-Linked Inheritance in Humans 22 pairs of autosomes, 1 pair of sex chromosomesX and YIn females, the sex chromosomes are XXIn males, the sex chromosomes are XYNote that in males the sex chromosomes are not homologousTraits controlled by genes in the sex chromosomes are called sex-linked traitsX chromosome has many genes, the Y chromosome does not
38 Sex-Linked Alleles Red-green colorblindness is X-linked The X chromosome has genes for normal color visionXB = normal visionXb – colorblindnessGenotypes PhenotypesXBXB female with normal color visionXBXb carrier female with normal color visionXbXb colorblind femaleXBY male with normal color visionXbY colorblind male
40 Polygenic Inheritance Occurs when a trait is governed by two or more sets of alleles.Each dominant allele codes for a productThe effects of the dominant alleles are additive.The result is continuous variation.Examples of traits include size or height, shape, weight, and skin color.
42 Environmental Influences Environmental factors can influence the expression of genetic traits.Example:Siamese cats and Himalayan rabbits are darker in color where body heat is lost to the environment.
43 Inheritance of Linked Genes All the alleles on one chromosome form a linkage group.Recall that during meiosis crossing over sometimes occursIf crossing over occurs between two alleles of interest, then four types of gametes are formed instead of two
45 The occurrence of crossing-over can help determine the sequence of genes on a chromosome Crossing-over occurs more often between distant genes than genes that are close togetherIn the example below, it is expected that recombinant gametes would include G and z more often than R and s.