Inheritance Topic 10.2 Biology HL
Dihybrid crosses
➢ Mendel also studied d
ihybrid crossings
○ Looking at two different traits at the same time in the same crossing
○ Example: round (R) or wrinkled (r) seed, yellow (Y) or green (y) seed
➢ All alleles could be shuffled in any random order
○ 16 random combinations
➢ As long as two traits are not linked, they should s
egregate independently
○ Able to pass onto the next generation either with or without the other
Autosomes and sex chromosomes
➢ T. H. Morgan found something different
○ Looked at eye colour of fruit flies (Drosophila)
■ Red is dominant
■ White is recessive
➢ The results were 100% red-eyed females and 100% white-eyed males
○ Ratio 1:1
➢ The first found sex-linked gene
○ The gene for eye colour in fruit flies has its locus on the X chromosome
Linkage groups
➢ Any two genes that are found on the same chromosome are said to be linked to each
other
○ Linked genes are usually passed onto the next generation together
➢ A group of genes inherited together because they are found on the same
chromosome are considered to be members of a l inkage group
○ Applies to genes found on both autosomes and sex chromosomes
Linked genes
➢ In the fruit fly the gene for body colour (gray (G) or black (g)) is in the same linkage
group as the gene for wing length (long (L) or short (l))
○ Linked genes are the genes that are on the same chromosomes
○ True breeding (homozygous) parents’ genotypes: GGLL x ggll
○ In order to show linkage, the following notation is used
G L g l G L
===== x ===== ⇒ GgLl ======
G L g l g l
1
, Inheritance Topic 10.2 Biology HL
○ Horizontal bars = homologous chromosomes
■ Show that the locus of G is on the same chromosome as L
➢ Sometimes crossing over can interfere with the linkage
➢ Outcomes of genetic crosses should typically follow Mandelian ratios of 3:1 for an
F2 monohybrid cross and 9:3:3:1 for a dihybrid cross
➢ There is nothing alarming about a slight variation, but if there is a significant
deviation it suggests that independent assortment is not happening and instead
the genes of the traits being observed are linked
Chi-squared tests (x2)
➢ A sample size is far smaller than the whole population studied and therefore the
ratios found in experiments are rarely equal to the theoretical ratios
➢ Chi-squared test is a statistical test that biologists use to see whether the
difference between an observed result and an expected result is statistically
significant
➢ The chi-squared value is compared to a table
○ The degrees of freedom is the number of data classes minus 1
○ In biology the probability 0.05/5% is used
➢ If the chi-squared value is lower than the c
ritical value the answer is within the
range for statistical significance
○ The null-hypothesis is therefore true
Polygenic inheritance
➢ Involves 2 or more genes influencing the expression of one trait
○ Note that monogenic traits are those depending on only one gene
○ Two or more allelic pairs are found in different loki increases the number of
possible genotypes
2
Dihybrid crosses
➢ Mendel also studied d
ihybrid crossings
○ Looking at two different traits at the same time in the same crossing
○ Example: round (R) or wrinkled (r) seed, yellow (Y) or green (y) seed
➢ All alleles could be shuffled in any random order
○ 16 random combinations
➢ As long as two traits are not linked, they should s
egregate independently
○ Able to pass onto the next generation either with or without the other
Autosomes and sex chromosomes
➢ T. H. Morgan found something different
○ Looked at eye colour of fruit flies (Drosophila)
■ Red is dominant
■ White is recessive
➢ The results were 100% red-eyed females and 100% white-eyed males
○ Ratio 1:1
➢ The first found sex-linked gene
○ The gene for eye colour in fruit flies has its locus on the X chromosome
Linkage groups
➢ Any two genes that are found on the same chromosome are said to be linked to each
other
○ Linked genes are usually passed onto the next generation together
➢ A group of genes inherited together because they are found on the same
chromosome are considered to be members of a l inkage group
○ Applies to genes found on both autosomes and sex chromosomes
Linked genes
➢ In the fruit fly the gene for body colour (gray (G) or black (g)) is in the same linkage
group as the gene for wing length (long (L) or short (l))
○ Linked genes are the genes that are on the same chromosomes
○ True breeding (homozygous) parents’ genotypes: GGLL x ggll
○ In order to show linkage, the following notation is used
G L g l G L
===== x ===== ⇒ GgLl ======
G L g l g l
1
, Inheritance Topic 10.2 Biology HL
○ Horizontal bars = homologous chromosomes
■ Show that the locus of G is on the same chromosome as L
➢ Sometimes crossing over can interfere with the linkage
➢ Outcomes of genetic crosses should typically follow Mandelian ratios of 3:1 for an
F2 monohybrid cross and 9:3:3:1 for a dihybrid cross
➢ There is nothing alarming about a slight variation, but if there is a significant
deviation it suggests that independent assortment is not happening and instead
the genes of the traits being observed are linked
Chi-squared tests (x2)
➢ A sample size is far smaller than the whole population studied and therefore the
ratios found in experiments are rarely equal to the theoretical ratios
➢ Chi-squared test is a statistical test that biologists use to see whether the
difference between an observed result and an expected result is statistically
significant
➢ The chi-squared value is compared to a table
○ The degrees of freedom is the number of data classes minus 1
○ In biology the probability 0.05/5% is used
➢ If the chi-squared value is lower than the c
ritical value the answer is within the
range for statistical significance
○ The null-hypothesis is therefore true
Polygenic inheritance
➢ Involves 2 or more genes influencing the expression of one trait
○ Note that monogenic traits are those depending on only one gene
○ Two or more allelic pairs are found in different loki increases the number of
possible genotypes
2
