GB 18- Human Genetics

Human Genetics

• Study of inheritance 

difficulties studying human genetics

•  Few offspring 

• Long generations 

• Can’t experiment on humans 

How to study human genetics

• Population studies of large extended families 

• Analyze matings that have already occurred, rather than manipulating (natural experiment) 

• DNA sequencing- many generations, extended relatives, association studies 

pedigree

• family tree that shows inheritance over several generations

• Can determine inheritance pattern

• Based on prevalence and distribution of trait 

• Often used in genetic counseling 

Human Traits with Mendelian Inheritance: 2 types

1. Autosomal Recessive: to hv the disorder you must be homozygous recessive and 2 copies of the mutation are required (parents of those affected are usually carriers) ex: sickle cell disease

2. Autosomal Dominant: much rarer for disease to be autosomal dominant, only one allele is required for the trait and phenotypes often only apparent lae in life (after reproduction) ex: polydactyly, even heterozygous individuals are affected in autosomal dominant

• In general: genes—>?—>?

• Sometimes: mutated allele—> ?—> ? (mutation can be ——- or ——-, usually —— though) 

• In general: genes—>proteins—>function 

• Sometimes: mutated allele—> non functional or absent protein—> no function (mutation can be recessive or dominant, usually recessive though) 

disomy

• having 2 copies of each chromosome (1 homologous pair) 

• typical state

Aneuploidy

• presence of atypical number of a particular chromosome 

• Extra or missing copy 

Trisomy

• having 3 of a chromosome 

• 2n +1 

Monosomy

• lacking 1 member of a pair of chromosomes

• 2n-1 

what is the reason for aneuploidy?

• Due to nondisjunction: 

• Most common in anaphase 1 

Aneuploidy viability: 

• In humans, monosomies for all autosomes inviable (unsuccessful/ will not survive)

• Most trisomies also inviable 

• Estimated that >50% of pregnancies end before 8 weeks 

• Down syndrome is the most common because it's one of the only ones that could survive 

Aneuploidy of sex chromosomes: 

• Differences in number of sex chromosomes relatively common 

• Many sex chromosomes aneuploidies are viable but infertile 

• Common: XO, XXY, XYY 

Changes to chromosome structure 

• Due to errors in meiosis, radiation, or other mutagens 

• 4 types: deletion, duplication, inversion, translocation 

Insertions

duplicate a part of a chromosome segment, resulting in extra genetic material.

deletion

loose a part of a chromosome segment, leading to the loss of genetic material.

inversion

change in orientation of a chromosome segment, where a section of DNA is reversed end to end, potentially disrupting gene function.

translocation

piece of one chromosome breaks off and attached to different chromosome

Fetal testing

• genetic tests done before birth 

• Amniocentesis 

• Chloric villus sampling (CVS) 

• Non invasive prenatal screening/testing (blood work) (can be done early)

Newborn Screening 

• Some genetic disordered detected at birth 

• Example: metabolic disorders 

• Example: phenylketonuria (PKU) 

• Every newborn in US tested for PKU