The document discusses the chromosomal basis of inheritance, focusing on patterns for autosomal and sex-linked genes, linked genes, and the impact of incomplete dominance and codominance. It describes genetic disorders arising from nondisjunction, such as Down's syndrome and Turner syndrome, as well as the unique inheritance patterns of sex-linked traits. Key points include the relationship between chromosome behavior during meiosis and genetic variation, along with the implications of chromosomal alterations on heredity.

1. Chromosomal Basis of Inheritance
Ayesha safdar
17130814-029
Animal genetics
Msc-2nd

2. Objectives
 Be familiar with patterns of inheritance for autosomal and sex linked
genes
 Understand the concept of “Linked Genes”
 Understand how traits affected by incomplete dominance and
codominance differ from autosomal dominant and autosomal
recessive traits
 Understand how nondisjunction of chromosomes can lead to
disorders.

3. Linked Genes
Linked genes: are those that reside on the same chromosome
and tend to be inherited together
Humans have 23 pairs of chromosomes
 Autosomal genes reside on the autosomal
chromosomes (pairs 1-22)
 Sex-linked genes are found on the sex chromosomes
(pair 23, usually on the X)

4. Karyotype:
Picture of a complete set of chromosomes
picture of a complete set of chromosomes

5. Autosomal Genes
Autosomal genes
are usually
represented by a
pair of alleles
The phenotype of
the gene reflects the
dominant or
recessive
relationship of the
alleles.

6. Autosomal Disorders
Most autosomal genetic diseases are autosomal recessive meaning the individual
need to be homozygous recessive to exhibit the condition
(example: cystic fibrosis) Production of abnormmaly thick mucus. Leading to the
blockage of panreatic duct, intestines and respiratory infection.
Huntington’s disease is an autosomal dominant disorder meaning that is a single
Huntingtons allele is inherited, the individual will have the disease.

7. Incomplete and Codominance
Some alleles do not show a dominance hierarchy
Incomplete dominance: the phenotype of a
heterozygous genotype is intermediate in
appearance
Codominance: each allele in the genotype for a
particular gene will be expressed in the
phenotype

8. Incomplete Dominance:
Heterozygous genotype is intermediate in appearance.

9. Codominance:
Each genotypic allele will be expressed.

10. Sex Linked Inheritance
 Males and females differ in
their sex chromosome
combination
(females XX; males XY)
 Because the X contains
genes and the Y “does not”,
inheritance patterns of sex-
linked genes vary between
the sexes
recessive traits more
prevalent in males

11. Is the Disorder an Autosomal Recessive Disorder or Is It
Sex Linked?

12. Is This Disorder Autosomal Recessive or Sex
Linked?

13. Errors In Chromosomal
Inheritance
 Genetic disorders can
also occur due to
errors in the number
of inherited
chromosomes
 This condition arises
through a problem
that occurs during
meiosis

14. Nondisjunction
 Failure of a pair of
chromosomes to
separate during
gamete formation
 Genetic disorders like:
– Down’s syndrome
– Turner syndrome (XO)
– Metafemale (XXX)
– Kleinfelter Male (XXY)
– Jacob’s Male (XYY)

15. X inactivation in female mammals
 Although female mammals, including humans,
inherit two X chromosomes, one X
chromosome in each cell becomes almost
completely inactivated during embryonic
development.
o Barr body

16. Barr Body:

17. Abnormal Chromosome
•Nondisjunction:
–Leads to aneuploidy:
•Aneuploidy: is the condition of having less
than or more than the normal diploid number of
chromosomes, and is the most frequently
observed type of cytogenetic abnormality.

18. Alterations of chromosome
structure

19. The Key Points in Review
 Mendelian inheritance has its physical basis in the
behavior of chromosomes during sexual life cycles.
 Morgan traced a gene to a specific chromosome.
 Sex-linked genes have unique patterns of
inheritance.
 Alterations of chromosome numbers or structure
cause some genetic disorders.

20. Key Points in Review
 Linked genes tend to be inherited together
because they are located on the same
chromosome.
 Independent assortment of chromosomes and
crossing over produce genetic variation
(recombinants)
 Geneticists can use recombination data to map a
chromosomes genetic loci.
 Chromosomal basis for sex is dependent upon the
organism.

21. Thank you