Biology Reading Notes
October 2, 2012
11.1 Genetic Linkage and Recombination
- Gregor Mendel perform experiments called crosses or test crosses with peas,
involving 7 different characteristics and genes that are unique to that plant.
- Each of the genes sorted independently of all others.
- Genes on the same chromosome are known as linked genes and the phenomenon is
- Following meiosis I and II, each of the four different chromatids will become a
chromosome in a separate gamete.
- Mutant alleles are named based on the altered phenotype of the organism that
- The names for dominant mutant alleles are written with the ﬁrst letter in Uppercase,
whereas those for recessive mutant alleles are written in lowercase.
- Genes consist of sequences of nucleotides in DNA and are arranged linearly in
- Genes carried on the same chromosome are linked together in their transmission from
parent to offspring. Linked genes are inherited in patterns similar to those of single
genes, except for changes in the linkage due to recombination.
- As a result of recombination, the order of a particular collection of alleles linked on any
given chromosome is mixed up as a result of exchange with corresponding alleles on
the other homologous chromosome. The exchanges occur while homologues pair
during prophase I of meiosis.
- The likelihood of recombination between any two genes located on the same
chromosome pair reﬂects the physical distance between them on the chromosome.
- The greater this distance, the greater the chance will be that chromatids will exchange
segments at points between genes and the greater the frequency of recombinant
products of meiosis. (Gametes in animals, spores in plants)
- The relationship between separation and recombinant offspring frequencies is used to
produce chromosome maps in which genes are assigned relative locations with
respect to each other.
- Test crosses (AaBb x aabb) can used to detect linkage. If all progeny classes are
equally frequent, then the genes are not linked.
- Genes carried on the same chromosome may not show genetic linkage if they are
quite far apart.
What type of cross is typically used to discover whether two genes are linked or not?
A test cross is used to determine if two genes are linked. How can two genes be on the same chromosome and yet assort independently (as if
they were on separate chromosomes)?
Morgan concluded that characters remained together because they were
physically near to one another on the chromosome and were less likely to exhibit de
Vriesʼ factor exchange. Morgan called this within chromosome recombination crossing-
over when they were far apart. Further, he postulated that the closer in proximity that
the two genes were to one another, the more frequently they would be observed to
remain associated together.
11.2 Sex-Linked Genes
- Sex chromosomes are different in males (XY) than they are in females (XX)
- Genes located on these sex chromosomes are called sex linked genes.
- Chromosomes that are not sex-chromosomes are called autosomes. - Genes on
these chromosomes have the same patterns of inheritance in both sexes. In humans,
chromosomes 1-22 are autosomes.
- In most species with sex chromosomes, females have two copies of a chromosome
known as the X chromosome - forming a fully homologous XX pair whereas, males
have only one X chromosome.
- The Y chromosome occurs only in males and not in females.
- Each normal gamete produced by an XX female carries an X chromosome.
- Half the gametes produced by a male carry the Y chromosome and half carry the X
- One gene carried on the Y chromosome, SRY(for sex determining region of Y)
appears to be the master switch that directs development towards maleness at an
early point in embryonic development.
- Although X and Y chromosomes are called sex chromosomes, only a few genes that
carry have any inﬂuence on sex determination or sexual function.
- Since males and females have different sets of sex chromosomes, the genes carried
on these chromosomes can be inherited in a distinctly non-Mendelian pattern called
- Sex linkage arises from two differences between males and females: (1) males have
one X chromosome and there fore one allele for each gene on this chromosome;
females have two copies of the X chromosome and therefore two alleles for all the
genes on the X chromosome; (2) males also have one copy of the Y chromosome and
one allele for each gene on this chromosome; females have no Y chromosome and
therefore no Y alleles at all. Y chromosomes are present in males but not females.
- The X chromosome of each male comes from his mother.
- Pedigree: a chart that summarizes all parents and offspring for as many generations
- Sex linked recessive traits appear more frequently in males rather than females
because males need to receive only one copy of the allele on the X chromosome
inherited from their mother, whereas the females must receive two copies of the
recessive allele, one from each parent to express the trait. - The inactive, condensed X chromosome can be seen within the nucleus in cells of
females as a dense mass of chromatin called the Barr Body.
- Sex linkage is a pattern of inheritance produced by genes carried on sex
chromosomes: chromosomes that differ between males and females.
- Sex-linked inheritance patterns arise because, in humans and fruit ﬂies, females have
two copies of the X chromosome and therefore two alleles for each gene.
- Males have only one copy of the X chromosome and one of the Y chromosome and
therefore only have on allele for each gene.
- Only males have an allele for genes carried on the Y chromosome.
- Sex linkage is suggested by a particular, non-Mendelian pattern of inheritance when
the progeny of reciprocal crosses are different.