Genetics Chapter 17

Selective Breeding - choosing parents with particular characteristics to breed together and produce offspring with more desirable characteristics

• Selective breeding of wheat began as early as 9000 BC in the Middle East

• Farmers collected and planted the seeds of one variety of wheat more easily than the other, unknowingly “selecting for” this former strain

True Breeding- a kind of breeding wherein the parents would produce offspring that would carry the same phenotype

alleles- one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location

Phenotype - refers to how a trait is physically expressed

genotype - refers to the combination of alleles for a given trait

• Example of phenotype and genotype: Child's eye colour determined by dominant and recessive alleles from parents

Dominant allele controls the expressed characteristic

complete dominance - the effect of one allele in a heterozygous genotype completely masks the effect of the other

Recessive allele controls the repressed characteristic

Allele combinations- Traits are influenced by the interaction of two alleles, with one always dominant over the other

BB (homozygous dominant) results in a dominant phenotype

Bb (heterozygous) results in the dominant phenotype

bb (homozygous recessive) results in recessive phenotype

Homozygous - two identical alleles for a trait (RR or rr)

Heterozygous - Having two different alleles for a particular trait (Rr)

Gregor Mendel - first described the idea of dominant/recessive alleles

• Mendel conducted test crosses with peas and observed that offspring displayed either the maternal or paternal phenotype, not a blending of the two

• Mendel concluded that certain traits were dominant over others

  

Law of Segregation- Offspring display either the maternal or paternal phenotype, allowing for the random segregation of genes during gamete formation

Monohybrid Crosses- where only a single trait is considered

Chromosome theory of inheritance - the alleles that determine a trait are in the genes within specific chromosomes. This theory also explains how dominant and recessive alleles are transmitted from parent to child, and that each trait is transmitted independently.

MODE OF INHERITANCE

• Autosomal dominant mode of inheritance= the trait is dominant & inherited on an autosome (as opposed to the sex chromosomes)

• Autosomal recessive mode of inheritance= the trait is recessive & inherited on an autosome (as opposed to the sex chromosomes)

DIHYBRID CROSSES - crosses involving 2 genes

Law of independent assortment- The two alleles for one gene segregate independently of alleles for other genes during gamete formation

incomplete dominance - a condition where neither of the two alleles for the same gene have complete dominance over the other

Co-dominance - a situation where both alleles are fully expressed

Heterozygote - the result of incomplete dominance or co-dominance

sex-linked traits- genes that are located on the X or Y chromosome

Barr Bodies- the inactive X-chromosomes in females

Multiple Alles- often more than two alleles may exist for a particular trait (one organism can only have 2, but more than one can exist among a population creating variation)

Blood types- A, B, AB, or O

Epistasis- describes a certain relationship between genes, where an allele of one gene (e.g., 'spread') hides or masks the visible output. ex: The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus.

Pedigrees- a type of flowchart that uses symbols to show the expression of a particular trait through multiple generations of a single family.
Roman numerals are used to indicate generations. Numbers are used to indicate individuals within each generation

GENE LINKAGE - the inheritance of a particular trait is thought to follow Mendel’s law of independent assortment, allowing us to predict the phenotypic and genotypic ratios of offspring accurately.

Recombinant- An event where crossing over takes place and forms 2 distinct pairs of alleles from what the parents had.

Non-linked genes = 50% parental gametes, 50% recombinants

Linked genes = less than 50% recombinants

Gene Mapping-