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Chapter 9

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Department
Biology
Course
Biology 1225
Professor
Michael Butler
Semester
Fall

Description
Chapter 9 Observing Patterns in Inherited Traits Gregor Mendel wrote about “units of inheritance” we now know that genes are the fundamental units of inheritance. It is essential for you to make the connection between the movement of chromosomes in meiosis and patterns of inheritance of genes carried on them. Know the connection between Fig. 8.7 and 8.8. How does the movement of chromosomes during the production of gametes in both parents give rise to the given offspring? ***Do NOT study 9.5 and 9.8. NOT ON EXAM Look at Genetics problems You are required to be able to construct and understand the resulting genotypes and phenotypes and their ratios for a complete Punnet square for a Monohybrid and a Dihybrid cross. EX: construct the Punnet square for the monohybrid cross Aa x Aa 1 9.2 Tracking Traits pg. 151 Mendel started to study inheritance using the pea plant pg. 151 Mendel’s work tracking pea plant traits led him to conclude that hereditary information passes from one generation to the next. Look at Fig. 9.2 on page 151 purple flower peas crossed with white flower peas will give you only purple flower peas dominant – refers to an allele that masks the effect of a recessive allele in heterozygous individuals recessive – refers to an allele with an effect that is masked by a dominant allele in heterozygous individuals heterozygous – having 2 different alleles of a gene homozygous – having identical alleles of a gene genotype – the particular set of alleles that is carried by an individual’s chromosomes phenotype – An individual’s observable traits monohybrid cross – cross between two individuals identically heterozygous for one gene; Example Aa x Aa Punnett square – Diagram used to predict the genetic and phenotypic outcome of a cross 1. Individuals with identical alleles are homozygous for the allele 2. Heterozyous individuals have two non identical alleles 3. A dominant allele masks the effect of a recessive allele on the homologous chromosome 4. Genotype (an individual’s particular set of alleles) gives rise to phenotype (what the individual will look like 2 Take Home Message How do alleles contribute to traits? 1. Genotype refers to the particular set of alleles that an individual carries. Genotype is the basis of phenotype, which refers to the individual’s observable traits. 2. A homozygous individual has two identical alleles of a gene. A heterozygous individual has two non identical alleles of the gene 3. A dominant allele masks the effect of a recessive allele paired with it in a heterozygous individual. 3 9.3 Mendelian Inheritence Patterns pg 152 Dihybrid cross – Cross between two individuals identically heterozygous for two genes for example AaBb X AaBb Monohybrid cross – cross between two individuals identically heterozygous for one gene; for example Aa X Aa 1. Crossing individuals that breed true for two forms of a trait yields identically heterozygous offspring. : A cross between such offspring is a monohybrid cross. 2. The frequency at which the traits appear in the offspring of such crosses can reveal dominance relationships among the alleles associated with those traits. 3. Punnett squares are useful in determining the probability of the genotype and phenotype of the offspring of crosses. 4. Diploid cells have pairs of genes on homologous chromosomes. 5. The two genes of a pair separate from each other during meiosis, so they end up in different gametes. 6. Crossing individuals that breed true for two forms of two traits yields offspring that are identically heterozygous for alleles governing those traits : A cross between such offspring is a dihybrid cross. 7. Gene pairs on homologous chromosomes tend to sort into gametes independently of other gene pairs during meiosis. Take Home Message How do alleles contribute to traits? 1. Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of a pair (which may be identical or not) are separated from each other during meiosis, so they end up in different gametes 2. In most cases, the two genes of a pair are distributed into gametes independently of other gene pairs during meiosis. 4 9.4 Behond simple dominance pg. 156 codominance – effect in which two alleles are both fully expressed in heterozygous individuals and neither is dominant over the other ***Example
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