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OBJECTIVE 7

OBJECTIVE 7. Applying Mendel’s law to determine phenotypic and genotypic probabilities of offspring

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OBJECTIVE 7

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  1. OBJECTIVE 7 • Applying Mendel’s law to determine phenotypic and genotypic probabilities of offspring • Defining important genetic terms, including dihybrid cross, monohybrid cross, phenotype, genotype, homozygous, heterozygous, dominant trait, recessive trait, incomplete dominance, co-dominance, and allele • Interpreting inheritance patterns shown in graphs and charts • Calculating genotypic and phenotypic percentages and ratios using a Punnett square

  2. Heredity • The passing of traits from parents to offspring • Genetics – the study of how traits are passed from parents to offspring • Gregor Mendel – the “father of genetics” • Studied heredity in pea plants • Crossed a pea plant that produced purple flowers with a pea plant that produced white flowers – all of the offspring had purple flowers • Cross two of the purple-flowered offspring – some offspring had purple flower and some had white

  3. Monohybrid Crosses • A cross that involves ONE pair of contrasting traits • Ex. Crossing a plant with purple flowers and a plant with white flowers • True-breeding – all the offspring would display only one form of the character • P generation – parental generation (the first two individuals that are crossed in a breeding experiment • F1 generation – filial generation (offspring of the P generation • F2 generation – second filial generation (offspring of the F1 generation)

  4. Dihybrid Cross • a cross that involves TWO pairs of contrasting traits

  5. Mendelian Theory of Heredity • For each inherited character, an individual has two copies of the gene, one from each parent. • There are alternative versions of genes. • Allelles – the different versions of a gene

  6. 3. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. • Dominant – the expressed form of the character (the “stronger” form); represented by capital letter • Recessive – the trait that was not expressed when the dominant form of the character was present (the “weaker” form); represented by lowercase letter 4. When gametes are formed, the alleles for each gene in an individual separate independently from one another. Thus, gametes carry only one allele for each inherited character. When gametes unite during fertilization, each gamete contributes one allele.

  7. Genetic Terminology • Homozygous – two alleles for a particular gene in an individual are the SAME • Ex. RR or rr • Heterozygous – two alleles for a particular gene in an individual are DIFFERENT • Ex. Rr • Genotype – the set of alleles that an individual has for a character • Phenotype – the physical appearance of a character

  8. The Laws of Heredity • The Law of Segregation – the two alleles for a character segregate (separate) when gametes are formed • The Law of Independent Assortment – the alleles of different genes separate independently of one another during gamete formation

  9. Punnett Squares • A diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross • The possible gametes that one parent can produce are written along the top of the square • The possible gametes that the other parent can produce are written along the left side of the square • Each box inside the square is filled with two letter by combining the allele along the top of the box with the allele along the side of the box • The letters in the boxes show the possible genotypes of the offspring

  10. Probability • The likelihood that a specific event will occur • Can be expressed in words, as decimals, as percentages, or as fractions • Probability = number of one kind of possible outcome total number of all possible outcomes

  11. Inheritance of Traits • Pedigree – a family history that shows how a trait is inherited over several generations • Helpful if the trait causes a genetic disorder • Predicts carriers of the disorder • Carriers – individuals who are heterozygous for an inherited disorder but do not show symptoms of the disorder • Autosomal genes will appear in both sexes equally

  12. Sex-linked Traits • Usually only seen in males • A sex-linked gene’s allele is located only on the X chromosome. • Most are located on the X and are recessive • Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will have the sex-linked condition • A female who carries a recessive allele on one X chromosome will not have the condition if there is a dominant allele on her other X chromosome. She would be a carrier. Females must inherit two recessive alleles to inherit the trait.

  13. Incomplete Dominance • Neither trait is dominant over the other trait so a third blended trait appears • An organism displays a phenotype that is intermediate between the two parents • Examples: • Snapdragons: a red flower crossed with a white flower creates a pink offspring • Caucasians: a curly-haired parent and a straight-haired parent produced a child with wavy hair

  14. Codominance • Two dominant alleles are expressed at the same time • Both forms of a character are displayed • Example: AB blood type

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