Lecture-6
Chromosomal
Theory of
Inheritance
Learning Objectives

At the end of this session, the student should be able
to:
a)Define chromosome.
b)Describe the stages of mitosis.
c)Describe the stages of meiosis.
d)Describe Clinical Correlates.
Reference: Langman’s Medical Embryology, 11th edition,
pages 13-23.
What is a chromosome

In the nucleus of each cell, the DNA molecule is packaged
into thread-like structures called chromosomes. Each
chromosome is made up of DNA tightly coiled many times
around proteins called histones that support its structure.

Each chromosome has a constriction point called the
centromere, which divides the chromosome into two
sections, or “arms.” The short arm of the chromosome is
labeled the “p arm.” The long arm of the chromosome is
labeled the “q arm.”

The location of the centromere on each chromosome gives
the chromosome its characteristic shape, and can be used to
help describe the location of specific genes.
MITOSIS
 Stages:
 Prophase
 Pro-metaphase
 Metaphase
 Anaphase
 Telophase
 Daughter
cells
MEIOSIS

Cell division taking place in the germ
cells to generate male and female
gametes.

Meiosis requires two cell divisions,
meiosis I and meiosis II, to reduce the
number of chromosomes to the haploid
number of 23.

Meiosis I:

Replication of DNA (Duplicated 46 chromosomes).

Pairing of homologous chromosomes (synapsis).

Cross over---Chiasma formation.

Falling apart of double-structured chromosomes.

Anaphase

Result of Meiosis I: Two daughter cells with 23 double
structured chromosomes.

Meiosis II:

Longitudinal splitting of double structured
chromosomes at the centromere.

Result: The cells containing 23 single
chromosomes.
Meiosis-Results

Provides constancy of the chromosome number from
generation to generation by reducing the chromosome
number from diploid to haploid, thereby producing haploid
gametes.

Allows random assortment of maternal and paternal
chromosomes between the gametes .

Relocates segments of maternal and paternal chromosomes by
crossing over of chromosome segments, which "shuffles" the
genes and produces a recombination of genetic material .
Abnormal Gametogenesis

Disturbances of meiosis during gametogenesis,
e.g., nondisjunction, result in the formation of
chromosomally abnormal gametes. If involved in
fertilization, these gametes with numerical
chromosome abnormalities cause abnormal
development such as occurs in infants with Down
syndrome
Clinical
Correlates
Chromosomal abnormalities

Numerical

Normal somatic cells-diploid (2n).

Normal gametes-haploid (1n).

Euploid= Exact multiple of “n” (dilpoid or triploid).

Aneuploid= Any chromosome number which is not euploid-extra
chromosome is present (trisomy)or one chromosome is
missing (monosomy).

Nondisjunction

Translocation: Chromosomes break and pieces of one
chromosomes attach to other.

Trisomy 21 (Down syndrome)

Trisomy 18 (Edward Syndrome)

Trisomy 13 (Patau Syndrome)

Klinefelter’s syndrome

Turner syndrome

Triple X syndrome
DOWN SYNDROME
TRISOMY 18 and TRISOMY 13
TRISOMY 18
Photograph of child with trisomy 18. Note
the prominent occiput, cleft lip,
micrognathia, low-set ears, and one or
more flexed fingers.
TRISOMY 13
A. Child with trisomy 13. Note the cleft lip and palate,
the sloping forehead, and microphthalmia.
B. The syndrome is commonly accompanied by
polydactyly.
KLINEFELTER SYNDROME
TURNERS SYNDROME
 Structural
 Deletion
abnormalities:
(Cri-du-chat syndrome)
 Microdeletions
 Angelman’s
syndrome
 Prader-Willi
syndrome
 Fragile
sites
CRI DU CHAT SYNDROME
PRADER WILLI SYNDROME
ANGELMAN’S SYNDROME