Sturtevant proposed that the farther apart linked genes were on a chromosome, the greater
the chance that non-sister chromatids would cross over in the region between the genes
during meiosis. By determining the number of recombinants - offspring in which a cross-over
event has occurred - it is possible to determine the approximate distance between the genes.
This distance is called a genetic map unit (m.u.), or acentimorgan, and is defined as the
distance between genes for which one product of meiosis in 100 products is a recombinant.
So, a recombinant frequency of 1% (1 out of 100) is equivalent to 1 m.u. Loci with a
recombinant frequency of 10% would be separated by 10 m.u. The recombination frequency
will be 50% when two genes are widely separated on the same chromosome or are located on
different chromosomes. This is the natural result of independent assortment. Linked genes
have recombination frequencies less than 50%.
Determining recombination frequencies between genes located on the same chromosome al-
lows a linkage map to be developed. Linkage mapping is critical for identifying the location
of genes that cause genetic diseases.
Lesson Summary
- Variants of genes are called alleles.
- Genotype is the combination of alleles that an individual has for a certain gene, while
phenotype is the appearance caused by the expression of the genotype. - Incomplete dominance and codominance do not fit Mendel’s rules because one allele
does not entirely mask the other. - In polygenic inheritance, many genes control a trait with each dominant allele having
an additive effect.
Review Questions
- What is a variant of a gene that occurs at the same place on homologous chromosomes?
- What is the type of allele that only affects the phenotype in the homozygous condition?
- What type of allele masks the expression of the recessive allele and is therefore ex-
pressed in the heterozygote? - What is the term for the specific alleles of an individual for a particular trait?
- What is the term for the appearance of the organism, as determined by the genotype?
- If a organism has a certain phenotype, such as a tall pea plant, does that mean it must
have the same genotype? - Whatistheterm forthepatternofinheritancewhere anindividualhasanintermediate
phenotype between the two parents? - IQ in humans varies in humans with most people having an IQ of around 100, and
with a few people at the extremes, such as 50 or 150. What type of inheritance do you
think this might describe? - A dark purple flower is crossed with a white flower of the same species and the offspring