The Chromosomal Basis of Inheritance

INHERITANCE AND CHROMOSOMES

• Parallels between chromosomes and Mendel’s proposed hereditary factors (genes):

  • both are present in pairs in diploid cells

  • homologous chromosomes separate and alleles segregate during the process of meiosis

  • fertilization restores the paired condition for both 

CHROMOSOME THEORY OF INHERITANCE

• Specific genes have specific loci along chromosomes

• it is the chromosomes that undergo segregation and independent assortment

CHROMOSOMAL BEHAVIOR

• Behavior of homologous chromosomes during Anaphase I accounts for Law of Segregation

• Behavior of non-homologous chromosomes during Metaphase I accounts for Law of Independent Assortment

SEX CHROMOSOMES

• Eggs always carry one X chromosome

  • could be paternal X

  • could be maternal X

• Sperm will always carry one or the other

  • ½ will carry X

  • ½ will carry Y

INHERITANCE OF X-LINKED GENES

• XX demonstrates normal inheritance patterns

• XY will present whichever allele is inherited

  • more XY individuals have x-linked recessive disorders

X-INACTIVATION

XX Individuals

One X-chromosome is methylated and shut down; it is called the Barr Body (epigenetic modification)

They exhibit a mosaic of X traits -- about half of the cells will exhibit the paternal traits and half of the cells will exhibit the maternal traits (if heterozygous, this could present as a patchy phenotype)

NONDISJUNCTION

In MEIOSIS I:

• Members of a pair of homologous chromosomes do not move apart separately during Meiosis I

• All 4 gametes have an error

In MEIOSIS II:

• Sister chromatids fail to separate during Meiosis II

• 2 of the 4 gametes have an error

• 2 of the 4 gametes are normal

ANEUPLOIDY

• Aneuploidy -- gametes with an abnormal number of a particular chromosome

  • Monosomic -- zygote is missing one chromosome (2n-1)

  • Trisomic -- zygotes has an extra chromosome (2n+1)

• Polyploidy -- more that two complete chromosome sets in all somatic cells

  • triploidy = 3n

  • tetraploidy = 4n

Imprinting & Organelle Genes

Certain genes are methylated depending on if in an egg or a sperm.  Once the zygote is created, it will erase all the imprints and re-imprint based on if they are creating eggs or sperm.

Organelle genes follow one parent or the other.  

In humans, the egg is responsible for the mitochondrial lineage.

In plants, often times the mitochondria is inherited through the male parent and the chloroplast is inherited through the female parent.