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mendel & the gene idea (mendel used the scientific approach to…
mendel & the gene idea
mendel used the scientific approach to identify two laws of inheritance
gregor mendel formulated a theory of inheritance based on experiments with garden peas, proposing that parents pass on to their offspring discrete genes that retain their identity through generations. this theory includes two "laws"
the law of segregation states that genes have alternative forms, or allels. in a diploid organism, the two allels of a gene segregate (seperate) during meiosis and gamete formation; each sperm or egg carries only one allele of each pair.
this law explains the 3:1 ratio of F2 phenotypes observed when monohybrids self-pollinate. each organism inherits one allele for each gene from each parent
monohybrids- an organism that is heterozygote with respect to a single gene of interest. all of the offspring from a cross between parents homozygous for different alleles are monohybrid
for example, parents of genotypes AA and aa produce a monohybrid of genotype Aa
law of segregation- mendel's first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation
allels- any of the alternative versions of a gene that may produce distinguishable phenotype effects
in heterozygotes, the two alleles are different; expression of the dominant allele masks the phenotypic effect of the recessive allele.
heterozygotes- an organism that has two different alleles for a gene (encoding a character)
dominant allele- an allele that is fully expressed in the phenotype of a heterozygote
recessive allele- an allele whose phenotypic effect is not observed in a heterozygote
homozygotes have identical allels of a given gene and are therefore true-breeding
homozygotes - an organism that has a pair of identical alleles for a gene (encoding a character)
true-breeding - referring to organisms that produce offspring of the same variety over many generation of self-pollination
the law of independent assortment states that the pair of allels for a given gene segregates into gametes independently of the pair of alleles for any other gene.
law of independent assortment- mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes
in a cross between dihybrids the offspring have four phenotypes in a 9:3:3:1 ratio
dihybrids - an organism that is heterozygous with respect to two genes of interest. all the offspring from a cross between parents doubly homozygous for different alleles are dihybrids
for example is parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb
ex: individuals heterozygous for two genes
probability laws govern medelian inheritance
the multiplication rule states that the probability if two or more events occurring together is equal to the product of the individual probabilities of the independent single events
multiplication rule- a rule of probability stating that the probability of two or more independent events occurring together can be determined by multiplying their individual probabilities
the addition rule states that the probability of an event that can occur in two or more independent, mutually exclusive ways is the sum of the individual probabilities
the addition rule- a rule of probability stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities
the rules of probability can be used to solve complex genetics problem. a dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously. in calculating the chances of the various offspring
inheritance patterns are often more complex than predicted by simple mendelian generics
relationships among alleles of a single gene; description; examples
complete dominance of one allele; heterozygous phenotype same as that of homozygous dominant; PP Pp
incomplete dominance of either allele; heterozygous phenotype intermediate between the two homozygous phenotypes; CR CR, CR CW, CW CW
codominance; both phenotypesmexpressed in heterozygotes; IA IB
multiple alleles; in the population, some genes have more than two allels; ABO blood group allels IA IB i
pieiotrophy; one gene affects multiple phenotypic characters; sickle-cell disease
relationship among two or more genes
epistasis; the phenotypic expression of one gene affects the expression of another gene; BbEe x BbEe
polygenic inheritance; a single phenotypic character is affected by two or more genes; AaBbCc x AaBbCc
many human traits follow mendelian patterns of inheritance
analysis of family pedigrees can be used to deduce the possible genotypes of individuals and make predictions about future off-spring. such predictions are statistical probabilities rather than certainties
many genetic disorders are inherited as simple recessive traits. most affected (homozygous recessive) individuaos are children of phenotypically normal, heterozygous carriers
carriers - in genetics, an individual who is heterosygous at a given genetic locus for a recessively inherited disorder. the heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring
the sickle-cell allele has probably persisted for evolutionary reasons: homozygotes have sickle-cell disease, but heterozygotes have an advantage because one copy of the sickle-cell allele reduces both the frequency and severity of malaria attacks