Chromosomal Inheritance and Human Heredity

Human Chromosomes Karyotype – a picture of an organism’s chromosomes We take pictures during mitosis when the chromosomes are condensed Then you cut out the pictures and group the like chromosomes together Can be used to determine sex and some genetic disorders

Karyotypes

A normal human karyotype contains 46 chromosomes or 23 pairs Pairs 1-22 – autosomes Pair 23 – sex chromosomes XX female XY male

Pedigrees Can be used to trace traits through families Can be analyzed to determine the genotypes of family members Uses symbols to represent family members  Female Horizontal line - mating  Female with traitVertical line - offspring  Male Male with trait  Carrier (half filled in)

Pedigrees

Human genes Genome – complete set of genetic information Includes tens of thousands of genes Human Genome Project – determined sequence of the human genome Completed in 2000 Helped to identify individual, specific genes

Blood Type Some of the first genes to be identified Determined by several genes Two most common are the ABO blood groups and the Rh blood groups

Rh Blood Group Determined by a single gene Two alleles – positive and negative Discovered in the Rhesus monkey “Rh” Positive allele is dominant to the negative allele Rh positive results in a protein in the blood

ABO Blood Group Single gene with three different alleles A and B are codominant (I A, I B ) O is recessive (i) Antigen – protein found on surface if blood cell Blood type A has A antigen, type B has B antigen Antibodies – made by immune system to fight foreign substances Are specific to antigens

ABO Blood Groups

Donating and Receiving Blood If a corresponding antigen and antibody meet, it can cause blood to agglutinate or clump up Because O has neither A or B antigens it is called the “universal donor” Because AB has no antibodies, it is called the “universal receiver”

Sex-Linked Genes Genes located on the sex chromosomes (X and Y) Y is much smaller and contains only a few genes Because males only have one X, all X-linked alleles are expressed, even if they are recessive Many genetic disorders are linked to the X chromosome Colorblindness Hemophilia Duchenne Muscular Dystrophy

X inactivation In female mammals (XX), one of the X chromosomes in each cell becomes inactivated (randomly) Condenses into a Barr body Even out the dose of “X” genes in females Different X’s can be activated in each cell which can lead to a mosaic effect Calico cats

Genetic Recombination Production of offspring with new combinations of traits (inherited from both parents) Offspring with new combinations are called recombinants Those with combinations like parents are called parental types Caused by independent assortment and crossing over

Chromosomal Theory of Inheritance Genes have a specific loci (locus) on chromosomes It is the chromosomes that undergo segregation and independent assortment, not the individual alleles

Linked genes Genes that are located on the same chromosome Tend to be inherited together May not follow independent assortment Crossing over can affect the inheritance of linked genes Typically the closer the genes are, the less likely there will be crossing over May not get expected ratios

Genetic/Linkage Maps An ordered list of loci of genes along a particular chromosome Based on the idea that the farther away two genes are, the more points there are between them where crossing over can occur Percentage of crossing determines distance apart Map units – express relative distances of genes along a chromosome Equivalent to a 1% recombination frequency

Nonnuclear Inheritance Organisms also obtain DNA from their parents in mitochondria and chloroplasts Genes in this DNA responsible for some traits Do not follow Mendelian patterns In animals, mitochondrial DNA is transmitted by the egg and thus these traits are inherited from the mother

Mutations Any change in DNA Gene Mutations Point Mutations – involve changes in one or a few nucleotides in the DNA sequence Includes substitutions, insertions, and deletions Can affect the protein produced Called SNPs – single nucleotide polymorphisms

Chromosomal Mutations Involve changes in the number or structure of chromosomes Can change location of genes Can change number of copies of genes Include: Deletions – loss of all or part of chromosome Duplications – extra copies of all or parts Inversions – reverse directions of parts Translocations – part of chromosome breaks off and attaches to another

Mutations

Chromosome Disorders Can be caused when homologous chromosomes fail to separate in meiosis Called nondisjunction Can lead to fewer chromosomes or extra chromosomes Aneuploidy – means abnormal chromosome number Trisomy, monosomy Polyploidy – having extra sets of chromosomes Common in plants Typically fatal in animals

Nondisjunction

Examples: Down Syndrome – extra 21 st chromosome Turner’s Syndrome – Only one X chromosome Klinefelter’s Syndrome – Males have an extra X chromosome (XXY)