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Mendelian Patterns of Inheritance
Chapter 11 Mendelian Patterns of Inheritance
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11.1 Gregor Mendel Mendel carefully designed his experiments and gathered mathematical data Austrian monk,
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11.1 Gregor Mendel studied science & math (under Doppler), failed out of teacher college, entered monastery became Head Abbot, experimented with pea plants, kept records, and performed statistical analysis
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Blending Concept of Inheritance
11.1 Gregor Mendel Blending Concept of Inheritance breeders knew that both sexes contribute equally to a new individual however, they thought offspring were always of intermediate appearance compared to parents this idea is known as the blending concept of inheritance
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Mendel’s Experimental Procedure
11.1 Gregor Mendel Mendel’s Experimental Procedure Mendel chose to use the garden pea easy to cultivate short generation time normally self-pollinate could be cross-pollinated by hand (hybridization) Mendel’s starting peas were true-breeding (offspring were like parent plants and life each other)
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Fig. 11.2a Garden pea anatomy
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Mendel’s Experimental Procedure, cont.
11.1 Gregor Mendel Mendel’s Experimental Procedure, cont. Mendel studied seven traits in peas all were “either/or” characters trait: heritable feature, like flower color character: variant for a character, like purple or white Mendel kept careful records and applied laws of probability, resulting in particulate theory of inheritance
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Fig. 11.2b Garden pea traits
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11.2 Mendel’s Law of Segregation
Two factors for each trait segregate during gametogenesis Mendel chose to cross pea varieties that only differed in one trait (monohybrid cross) Would they blend when crossed? P generation: original parents F1 generation: first generation of offspring F2 generation: second generation
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11.2 Mendel’s Law of Segregation
Mendel performed reciprocal crosses in both cases, all F1 offspring resembled parents this contradicted blending theory Did the other trait disappear? Mendel allowed F1 to self-pollinate F2 had a mix of traits in a 3:1 ratio this was possible if each parent had two copies of a trait but only passed along one
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F-2 F-1 P-1 Example Tall Plant Short Plant 100% Tall 75% Tall,
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Fig Monohybrid cross
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11.2 Mendel’s Law of Segregation
each individual has two factors for each trait the factors segregate during gamete formation each gamete contains only one factor from each pair of factors fertilization gives each new individual two factors for each trait
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11.2 Mendel’s Law of Segregation
As Viewed by Modern Genetics alleles are alternate forms of a gene that control a trait the dominant allele masks expression of the other allele, called the recessive allele dominant allele written as an uppercase letter recessive allele written as a lowercase letter
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11.2 Mendel’s Law of Segregation
As Viewed by Modern Genetics, cont. alleles, cont. alleles occur on a homologous pair of chromosomes at a particular location called the gene locus (plural: loci) meiosis explains the law of segregation and why gametes have only one allele for each trait
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Fig. 11.4 Homologous chromosomes
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11.2 Mendel’s Law of Segregation
As Viewed by Modern Genetics, cont. homozygous organisms have two identical alleles for a trait heterozygous organisms have two different alleles at a gene locus genotype refers to an organism’s alleles phenotype refers to an organism’s physical appearance
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Table 11.1 Genotype vs. phenotype
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems determine which allele is dominant determine genotype of both parents determine various types of gametes for both parents Laws of Probability the laws of probability can be used to determine the likelihood of a particular outcome chance has no memory
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems, cont. Laws of Probability, cont. multiplicative law of probability: the probability of two or more independent events occurring together is the product of their chance of occurring separately example:
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems, cont. Laws of Probability, cont. additive law of probability: the chance of an event that can occur in two or more independent ways is the sum of the individual chances example:
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems, cont. The Punnett Square shows all possible combinations of alleles in offspring (hence, probability) if the number of offspring is large enough, the number of each type corresponds to the probability
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Fig Punnett square
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems, cont. The Punnett Square, cont. Example: Mendel’s P-1 tall X short TT X tt t t T Tt Tt 100% Tall (Not unlike what Mendel observed!) T Tt Tt
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11.2 Mendel’s Law of Segregation
One-Trait Genetics Problems, cont. The Punnett Square, cont. Example: Mendel’s F-1 heterozygous tall X heterozygous tall Tt X Tt T t TT tall Tt tt short 3/4 tall or 75% 1/4 short or 25%
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11.2 Mendel’s Law of Segregation
One-Trait Testcross used to determine if an organism with a dominant phenotype is heterozygous or homozygous. cross the unknown dominant with a recessive individual if any offspring are recessive, the unknown was heterozygous simple!
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Fig. 11.6a One-trait testcross
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Fig. 11.6b One-trait testcross
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11.3 Law of Independent Assortment
Each pair of factors segregates independently of the other pairs Mendel crossed true-breeding plants that differed in two traits (dihybrid cross) F1 plants showed both dominant characteristics Mendel observed four phenotypes among F2 plants, leading to his law of independent assortment
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11.3 Law of Independent Assortment
Each pair of factors segregates independently of the other pairs Mendel’s law of independent assortment each pair of factors segregate independently of the other pairs all possible combinations of factors can occur in the gametes
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Fig. 11A Mendel’s laws and meiosis
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11.3 Law of Independent Assortment
Each pair of factors segregates independently of the other pairs Two-Trait Genetics Problems Drosophila melanogaster is popular with genetics researchers easy to breed easy to observe mutant characteristics most frequent alleles in population called wild type
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Figs. 11.7 and 11.8 Dihybrid crosses
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11.3 Law of Independent Assortment
Two-Trait Genetics Problems, cont. Laws of Probability work the same way as before many offspring must be counted to achieve these probabilities you’ll count many offspring! Punnett Square works the same way, too, but with more squares (use FOIL)
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11.3 Law of Independent Assortment
Two-Trait Genetics Problems, cont. Two-Trait Testcross used to determine if an individual is homozygous dominant or heterozygous for either of the two traits cross unknown individual with an individual with the recessive phenotype
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Fig. 11.9 Two-trait testcross
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11.4 Human Genetic Disorders
Humans can be affected by a variety of recessive and dominant genetic disorders autosome: any chromosome other than a sex (X or Y) chromosome Patterns of Inheritance autosomal dominant disorders affect people with AA or Aa autosomal recessive disorders affect people with aa
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11.4 Human Genetic Disorders
Patterns of Inheritance, cont. a pedigree shows the pattern of inheritance for a particular condition females represented by circles, males by squares shaded circles and squares are affected individuals a line between a circle and square is a union a vertical line leads to a child
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11.4 Human Genetic Disorders
Patterns of Inheritance, cont. pedigree, cont. in pattern 1, parents are carriers see page 192 to learn to recognize different recessive and dominant patterns
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Fig. 11.10 Autosomal recessive pedigree
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Fig. 11.11 Autosomal dominant pedigree
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11.4 Human Genetic Disorders
Autosomal Recessive Disorders Tay-Sachs Disease most common in Hassidic Jews, nerve cells swell and die Cystic Fibrosis bad mucus in bronchial tubes and excess salt in sweat 1/20 whites is a carrier
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11.4 Human Genetic Disorders
Autosomal Recessive Disorders, cont. Phenylketonuria (PKU) lack enzyme to break down phenylalanine, an amino acid abnormal break down products can cause brain damage
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11.4 Human Genetic Disorders
Autosomal Recessive Disorders, cont. Sickle Cell Disease most common in blacks abnormal hemoglobin causes misshapen red blood cells sickle cell disease: homozygous sickle cell trait: heterozygous
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11.4 Human Genetic Disorders
Autosomal Dominant Disorders Neurofibromatosis severity of symptoms varies tumors form from coverings of nerves Huntington Disease symptoms appear in middle age convulsions and jerky movements, mental deterioration caused by protein that clumps in nerve cells
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Fig. 11.13 Huntington disease
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11.4 Human Genetic Disorders
Autosomal Dominant Disorders, cont. Achondroplasia form of dwarfism associated with defect in growth of long bones heterozygous; aa results in normal length limbs and AA is fatal
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11.5 Beyond Mendelian Genetics
Mendelian genetics does not explain all patterns of inheritance dominance is a consequence of the mechanism that determines phenotypic expression it is not based on one allele suppressing another at the DNA level complete dominance does not occur in all traits, there are exceptions….
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11.5 Beyond Mendelian Genetics
Incomplete Dominance heterozygote has an intermediate phenotype (but not blending inheritance) example: red + white 100% pink in some flowers can be explained by pigment production in humans, curly and straight hair, sickle cell disease, Tay-Sachs disease
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Fig. 11.14 Incomplete dominance
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11.5 Beyond Mendelian Genetics
Codominance both alleles are dominant heterozygote has full traits of both example: AB blood in humans Multiple Allelic Traits more than two alleles/trait each individual still has only 2 alleles example: human blood type A and B codominant O is recessive to both
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Fig. 11.15 Inheritance of blood types
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11.5 Beyond Mendelian Genetics
Polygenic Inheritance more than one pair of alleles/trait fhenotypes follow normal distribution example: many human traits height weight eye/skin/hair color
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Fig. 11.16 Polygenic inheritance
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Fig Height in humans
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11.5 Beyond Mendelian Genetics
Epistasis one gene interferes with another example: coat color in mice Environment and the Phenotype both genotype and environment affect phenotype (“nature vs. nurture”) relative importance is unknown examples: Himalayan rabbits, hydrangias, human skin color
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Epistasis
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Fig. 11.18 Coat color in rabbits
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11.5 Beyond Mendelian Genetics
Pleiotropy single gene affects more than one characteristic of an individual example 1: in tigers and Siamese cats, the gene that controls fur pigments can cause nerve defects leading to being cross-eyed example 2: sickle cell disease
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11.5 Beyond Mendelian Genetics
Other consider the following enzyme pathway: genotype C_P_ results in purple flowers genotypes ccP_, C_pp, and ccpp result in white flowers follows 9:7 ratio in F-1 cross Substrate A Intermediate B Pigment Enzyme C Enzyme P
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Traits interesting only to biology students and teachers
tongue rolling: dominant free ear lobes (dominant) over attached ear lobes widow’s peak (dominant) over no peak left thumb on top (dominant) over right thumb on top hitchhiker’s thumb (recessive) to normal thumb mid digital hair (dominant) over no mid digital hair sex-influenced trait: dominant in one sex and recessive in the other long ring finger dominant in men, long pointer dominant in women
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Genotype versus phenotype
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