Chromosomes and Human Inheritance Notes







Basic Chromosome Structure
o Long arm; Short arm; Centromere; Chromatids; Thousands of genes per chromosome
o Review Human Karyotype Facts

23 pairs = 46 total

Each pair is a homolog (each contains instructions for same proteins as the other, but possibly different versions or alleles for it)

22 autosomes

2 sex chromosomes

XX = Female

XY = Male

Y much smaller than X

Y determines gender
o Wildtype (Original, often most common) vs. Mutant Phenotype (Newer; often less common)
o Homogametic (XX - Females) vs. Heterogametic Sex (XY - Males)
o Pedigrees

Shaded = Trait vs. Unshaded = No Trait

Trait skips generation = Recessive Trait

Never skips and lots of shading = Dominant Trait

Trait shows up more often in a gender = Sex-Linked

Circles = Females

Squares = Males
Blood Types
o O = Original or Wildtype = Most common = No type = No antigen = Antibody vs. A & B = Universal Donor = Only receives from O
o A = First mutant = 2nd Most common (especially among northern Europeans) = Type A antigen = Antibody vs. B = Donates to A or AB = Receives
from A or O
o B = 2nd Mutant = Most recent mutant = 3rd Most Common (especially certain Asian races) = Type B antigen = Antibody vs. A = Donate to B or AB;
Receives from B or O
o AB = Combination of both mutants = rarest = Type A & B antigens = No antibodies = Donate to AB only = Universal receiver
Linked Genes
o Genes on the same chromosome that sometimes do not follow Mendel's law of independent assortment
o Breaking Linked Genes

Crossing over

More likely to cross over together if further apart on same chromosome.

If too close, linkage will be broken often leading to Genetic Recombination of what should be "linked" traits

Genetic Recombination = When offspring looks different than what they should if genes are linked

Parental Type = When offspring looks like parents as they should if the genes are linked

Recombinants = When offspring looks unlike parents as they should if genes are linked (means crossing over happened)

More recombinants = Smaller recombination frequency (how often it is crossed over together) = More crossing over breaking the link = Genes
are closer on chromosomes
Thomas Hunt Morgan
o Father of molecular genetics
o Studied sex-linkage and gene mapping
Chromosomal Mapping (Linkage; Cytogentic; Physical; Sequencing)
o Linkage Map

Based on recombination frequencies between different genes that should be linked

Genetic Map Unit (centimorgan; named after Morgan) = Recombination Frequency of 1%

The higher the frequency the further the gene is to another gene

Looking at the relationships between many genes you can map the whole chromosome
o Cytogenic Maps

Based on gene expression

Example: Genes on the same chromosome are often deactivated together
o Physical Map

Tagging the DNA at a known gene sequence to see where it lights up on a picture

Often done with radioactive isotopes
o Sequencing Map

Based on computer analysis of the actual human genome

Human genome project – all genes have been sequenced and are searchable on a computer
Hemizigous genes
o Only one allele for trait
o X-linked traits in males

Y chromosome has no instruction because it is smaller, so X is the only one that instructs on trait)

Leads to expression of trait more often in males since they cannot have a protective gene or be carriers (Example: Hemophilia)
o X-Inactivation

In females on of the X’s always deactivates to avoid double the proteins being made

That means that only one set of instructions is active

Deactivated X is crumbed up and called Barr Body
Mutations (Changes in DNA structure or sequence due to damage or errors in replication)
o Chromosomal (Alter entire structure of chromosome and often affect multiple genes)

Deletion (Piece of chromosome missing)

Duplication (Piece of chromosome doubles)

Inversion (Piece of chromosome detaches and reattaches upside-down)





Translocation (Piece of chromosome detaches and attaches to a non-homolog)
Non-disjunction (Chromosomes fail to separate properly during Meiosis)
 Aneuploidy (Abnormal number of chromosomes because of non-disjunction; Examples: 2n +1, which is one extra copy of something or 2n
-1, which is one less copy of something)
 Monsomy (1 copy of the chromosome, instead of 2)
 Trisomy (3 copies of the same chromosome, instead of 2)
 Polyploidy
 Abnormal number of copies of the entire set of chromosome
 Instead of 2n, cell is 3n, 4n, 5n, 6n, 8n, etc.
 That means every single chromosome instead of being in pairs, is in quartets for example (4n)
o Gene (Alter sequence of 1 gene; does not affect other genes)

Point mutations (Only one thing changes)

Frame shift (A mutation that causes the entire code to shift in an direction or another)

Substitution (One base replaces another that should be there, often a point-mutation: single substitution)

Insertion (An extra base is added – causes frame shift)

Deletion (A base is deleted – causes frame shift)
o Missense mutation (Any mutation that causes the DNA code to no longer code for necessary protein or function)
Extranuclear genes
o Genes not associated with chromosomes
o Plasmids in plants and bacteria
o Chloroplasts in plants and algae
o Mitochondria in eukaryotes
o Patterns of inheritance do not follow Mandel
o For example, all mitochondrial DNA is inherited from mother since only thing that comes from male game is DNA

This means all mitochondrial DNA comes from mothers only

With the exception of very small mutations we all share the same mitochondrial DNA

This hints at possible “Eve” effect – all came from same woman

Mitochondrial DNA may be a better way to differentiate races than standard DNA analysis which reveals we are all 99.999% the same.
Human Inheritance Patterns
o Many genes show epistasis & pleitropy relationships

Epistasis – One gene affects how another one is expressed

Pleitropy – One gene affects many traits
o Many traits depend on many genets (multifactorial or polygenetic) and genets have an “quantitative” or “additive” effect where the summation of all
creates the trait
o Many genes have multiple alleles, as opposed to just 2 (multiple allelic genes)
o The expression of many genes depends on certain environmental conditions (epigenetics)

Most traits can be altered by environment

Still many traits depend heavily on genetics

Twin studies and epigenetics
 Differences between identical twins raised in different environments have greater chances of being “environmental”
 Differences between fraternal twins raised in identical environments have greater chances of being “genetic”
Genetics Disorders (See separate handout)