1. 10.2 Inheritance
Essential idea: Genes may be linked or unlinked and are
inherited accordingly.
http://upload.wikimedia.org/wikipedia/c
ommons/4/46/Dividing_Cell_Fluorescenc
e.jpg
http://ies.rayuela.mostoles.educa
.madrid.org/deptos/dbiogeo/recu
rsos/Apuntes/ApuntesBioBach2/i
magenes/genetica/droso.png
2. Understandings
Statement Guidance
10.2 U.1 Gene loci are said to be linked if on the same chromosome.
10.2 U.2 Unlinked genes segregate independently as a result of meiosis.
10.2 U.3 Variation can be discrete or continuous.
10.2 U.4 The phenotypes of polygenic characteristics tend to show
continuous variation.
10.2 U.5 Chi-squared tests are used to determine whether the difference
between an observed and expected frequency distribution is
statistically significant.
3. Applications and Skills
Statement Guidance
10.2 A.1 Morgan’s discovery of non-Mendelian ratios
in Drosophila.
10.2 A.2 Completion and analysis of Punnett squares for
dihybrid traits.
10.2 A.3 Polygenic traits such as human height may also
be influenced by environmental factors.
10.2 S.1 Calculation of the predicted genotypic and
phenotypic ratio of offspring of dihybrid
crosses involving unlinked autosomal genes.
10.2 S.2 Identification of recombinants in crosses
involving two linked genes.
Alleles are usually shown side by
side in dihybrid crosses, for
example, TtBb. In representing
crosses involving linkage, it is
more common to show them as
vertical pairs,
10.2 S.3 Use of a chi-squared test on data from dihybrid
crosses.
4. 10.2 U1 Gene loci are said to be linked if on the same chromosome.
• Genes have specific locations (loci) on chromosomes
• Chromosomes segregate and assort independently
http://web.csulb.edu/~kmacd/361-6-Ch2.htm
5. 10.2 U2 Unlinked genes segregate independently as a result of meiosis.
• Mendel’s law of independent
assortment states allele pairs
separate independently from
other allele
pairs during gamete formation
(meiosis).
• Therefore, traits on different
chromosomes are transmitted
to the offspring independently
of traits on other
chromosomes.
• An exception to this rule is
linked genes
https://biologywarakwarak.files.wordpress.com/2011/12/crossingover-rec.jpg
6. 10.2 U3 Variation can be discrete or continuous.
• Discrete Variation is controlled by alleles of
a single gene or a small number of genes.
The environment has little effect. In this case
you either have the characteristic or you
don't. Cystic fibrosis is a good example for
this; either you have cystic fibrosis or you
don’t.
• Chi-squared calculations work well when
using examples with discrete variation
• Continuous Variation is a complete range of
phenotypes that can exist from one extreme
to the other. Height is an example of
continuous variation. Continuous variation is
the combined effect of many genes (known
as polygenic inheritance) and is often
significantly affected by environmental
influences. Skin color is another example of
continuous variation. http://articles.latimes.com/2013/nov/11/entertainment/la-et-
cm-princess-bride-disney-musical-20131111
7. 10.2 U4 The phenotypes of polygenic characteristics tend to show
continuous variation.
8. 10.2 U4 The phenotypes of polygenic characteristics tend to show
continuous variation.
9. Example: Human skin color
• This is controlled by as many as
6 genes each with its own
alleles.
• As the number of genes
increases the amount of
phenotypic variation increases.
• The alleles control the
production of melanin which is
a pigment that colors skin.
• In this example the calculation
is performed with 3 genes each
with 3 alleles. The cross is
between two individuals
heterozygous at both alleles
Allele Key
A= add melanin
a= no melanin added
B= adds melanin
b= no melanin added
10.2 U4 The phenotypes of polygenic characteristics tend to show
continuous variation.
http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgeninteract.html
10. 10.2 A.3 Polygenic traits such as human height may also be influenced
by environmental factors.
• Polygenic inheritance occurs when two or more genes control the expression of
a phenotype.
• As the amount of genes that control one trait increase, the number of phenotypes
increases.
• Each additional gene has an additive affect, increasing the phenotypes. This is called
continuous variation.
• Example is human height, which varies from person to person within the same race,
and varies between different races. Height shows continuous variation.
http://www.buzzle.com/img/articleI
mages/345051-59012-35.jpg
11. 10.2 A.1 Morgan’s discovery of non-Mendelian ratios in Drosophila.
Thomas Hunt Morgan
Drosophila melanogaster –
fruit fly
Fast breeding,
chromosomes (XX/XY)
Discovered sex-linked gene,
located on X or Y chromosome.
These genes characteristic are
not expressed in the same
ratios as traits located on other
chromosomes.
12. 10.2 U5 Chi-squared tests are used to determine whether the difference
between an observed and expected frequency distribution is statistically
significant.
• A chi-square test is a statistical test that
can be used to determine whether
observed frequencies are significantly
different from expected frequencies
• These statistical tests enable us to
compare observed and expected
frequencies empirically and to decide if
the results we see are statistically
significant. Statistical significance in this
case implies that the differences are not
due to chance alone, but instead may be
caused by other factors at work.
• The formula to the right is for a
chi-squared test
O = observed frequencies
E = expected frequencies
X2 =
13. 10.2 U5 Chi-squared tests are used to determine whether the difference
between an observed and expected frequency distribution is statistically
significant.
14. 10.2 U5 Chi-squared tests are used to determine whether the difference
between an observed and expected frequency distribution is statistically
significant.
15. Mendel's Second Law of Independent Assortment
• This law states that allele pairs separate independently during the
formation of gametes.
• Therefore, traits are transmitted to offspring independently of one
another
10.2 A.2 Completion and analysis of Punnett squares for dihybrid traits.
16. Phenotype Key:
Phenotypic ratio:
– 9 Smooth Yellow:
– 3 Smooth green:
– 3 Rough Yellow:
– 1 Rough Green
10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
17. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
18. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
19. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
20. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
21. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
22. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
23. 10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
24. 10.2 S.3 Use of a chi-squared test on data from dihybrid crosses.
• For an example let’s use Mendel’s results from his pea plant crosses
• When he did a dihybrid cross between two heterozygotes RrYy x RrYy, the expected
phenotypic ratio due to independent assortment would be 9:3:3:1. Look at the
chart below to see his actual results.
25. 10.2 S.3 Use of a chi-squared test on data from dihybrid crosses.
26. Chi-Square Analysis Practice
• Two true-breeding Drosophila are crossed: a normal-winged, red-
eyed female and a miniature-winged, vermillion-eyed male. The F1
offspring all have normal wings and red eyes. When the F1 offspring
are crossed with miniature-winged, vermillion-eyed flies, the
following offspring resulted:
– 233 normal wing, red eye
– 247 miniature wing, vermillion eye
– 7 normal wing, vermillion eye
– 13 miniature wing, red eye
• What type of conclusions can you draw from this experiment? Explain
your answer.
10.2 S.3 Use of a chi-squared test on data from dihybrid crosses.
27. 10.2 S.2 Identification of recombinants in crosses involving two linked
genes.
• Link genes are located on the same
chromosome and do not sort
independently
• Genes inherited together
• These genes do not show the
expected Mendelian ratios as
predicted by the Laws Independent
Assortment.
28. Linked Genes
10.2 S.2 Identification of recombinants in crosses involving two linked
genes.
