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Chromosome Theory of Inheritance

by | Feb 17, 2024

  • “Genes have specific loci along chromosomes & undergo segregation and independent assortment”
    • Chromosomal inheritance generates genetic variation in sexual reproduction.
    • Evidence: parallels between genes during meiosis and behavior of chromosomes
  • Autosomal Inheritance: gene is located on one of autosomes
  • Male and female equally likely to inherit the gene

H. Morgan:

  • Discovered sex-linkage of genes

Fly Case Study

  • Have mutations affecting body color and wing structure which are linked:
    • Normal (wild) body color is gray (B) while mutation is black (b)
    • The normal wing (V) and vestigial wings (v, small, nonfunctional)
  • Heterozygous would be BbVv with BV on one chromosome and bv on another
    • Since genes are linked and cant assort independently can only make BV and bv gametes

Linked Genes

  • Linked Genes: two genes found on the same autosomal chromosome and usually inherited together (not assorted independently)           
    • Linked genes sometimes crossover to seperate & create new allele combinations → allows natural selection to act on them
    • Goes against Mendel’s Law of Independent Assortment (every character inherited on its own and ASSUMES EVERY GENE IS FOUND ON A DIFFERENT CHROMOSOME)
  • When OBSERVED amounts do not match the EXPECTED offspring phenotype → genes must be linked
    • Expected: parental & recombinant type are equal
    • More than 50% offspring look the same as parent = genes are linked

Percent Recombination

  • “AKA rate of crossing over” and is directly proportional to the distance between the two linked genes                     Genes are farther apart → more likely to cross over
    • Genes are closer together → more likely to be inherited
  • Percent Recombination: add up the recombinants → divide by total number of offsprings
    • 50% RF is max, < 25% = genes are close
  • Divide percent recombination by 2 to get map units distance btwn. genes
    • (high map units = high percent recombination = independent assortment & crossing over more likely)

Sex Chromosomes

  • X chromosome is bigger and contains more genes than y                             
    • So most disorders are found on the X chromosome & less disorders passed father → son because of so few y-linked genes
  • Sex is determined by interactions of gene products
  • gene WNT4 needed for female gonad, XY egg with extra gene copy can develop female gonad
  • SRY Gene: found on Y, makes males male, directs development of male anatomical features
  • X chromosome contains genes for more than reproduction
    • Nervous system, light receptors in eyes (color blindness)

Inheritance of Sex-Linked Genes

  • Sex-Linked Genes: gene located on either sex chromosomes, either X-linked or Y-linked
  • Males and females inherit diff number of X chromosomes which results in pattern of inheritance
    • Females have two copies of sex-linked genes (23 homologous chromosomes) but males only get one copy (22 homologous chromosomes)
  • Father passes Y-linked alleles to all sons but NOT daughters; mother passes X to both
    • Son can ONLY receive X chromosome from mother
      • any male that gets recessive X-linked allele got it from mom and will express trait bcuz only need one copy of X
    • X-linked Disorders: caused by absence of gene on X chromosome locus which results in missing protein (Ex: hemophilia)

Abnormal Chromosome Number

  • Nondisjunction: occurs when chromosomes or chromatids fail to separate to opposite poles during meiosis or mitosis → daughter cells (or gametes) with extra or missing chromosomes                                             
  1. Meiosis: failure for two homologous chromosomes (maternal and paternal migrate along spindle fibers together) or two chromatids
  2. Mitosis: Failure of two chromatids to separate
  • Happens most often during embryonic development and results in mosaicism in which fraction of body cells (descendents) have extra or missing chromosome
  1. Polyploidy occurs if all chromosomes undergo meiotic nondisjunction and produce gametes with twice number
  • If polyploid gamete fertilized with similar gametes → polyploid zygote (common in plants)
  • Aneuploidy: genome with extra or missing chromosomes; usually caused by nondisjunction
    • Can result in monosomic zygotes (missing a chromosome) or trisomic (extra)
    • Mitosis will then spread abnormality; almost always lead to genetic disorders

X Inactivation

  • During embryonic development of female mammals, one of the 2 X chromosomes does not uncoil into chromatin.
  • Instead X-Inactivation occurs and one chromosome remains coiled as a compact body called barr body
    • Inactivation makes sure that only one type of protein is made
  • Barr body: inactive X chromosomes (most of genes not expressed or interact [in dom./rec manner] with other chromosome)
    • Thus only alleles on one active X chromosome are expressed
  • One of X chromosome randomly is inactivated through DNA methylation and removal of acetylation on histone structure
    • After X is inactivated, mitosis results in body cells with same inactive X
  • In a developed fetus, some groups with have one X inactivated and while others will have other
    • SO all cells in a female mammal are not functionally identical

x-Inactivation & Sex-Linked Genetic Disorders

  • If a trait is X-linked then a male produced from a homozygous mother will always express the trait
  • A carrier female (X^N X^n) should normally be normal because some of the cells will have activated X^N
    • But in (rare) case where all cells with X^N inactivated, carrier female should express same symptoms for trait as male (ex: color blindness)

Case Study: Calico Cats

  • Calico cats female bcuz heterozygotes inherit two X-linked alleles for hair color → some cells will express red and other black color
  • Males inherit only one x-linked allele controlling hair color                                                                                     

Genomic Imprinting 

  • Only one gene is expressed and the other is silenced
  • GOES AGAINST EVERYTHING: sometimes you only use the allele inherited from mom or from the dad, doesn’t deal with dominance
  • Occurs during gamete formation & results in one allele silenced thru methylation (not expressed in offspring)

                Reversed in gonads during meiosis

Inheritance of Organelle Genes

  • Inheritance of traits controlled by organelles are inherited only from mother since male gamete (pollen or sperm) delivers negligible cytoplasm
    • Mother has mutation in an organelle (ex: mitochondria) → passed to all kids
  • This Maternal Inheritance can trace specific genome from progeny back thru generation