INTRODUCTION TO LINKED GENES AND SEX LINKAGE H. Biology/ Ms. Kim

Locating Genes on Chromosomes Genes – Are located on chromosomes – Code for protein that determines specific traits. Figure 15.1

Gene Linkage Linked genes – Usually inherited together because located near each other on the SAME chromosome Genes closer together on the same chromosome are more often inherited together Each chromosome – Has 100’s or 1000’s of genes Sex-linked genes exhibit unique patterns of inheritance; genes linked on the X or Y chromosome

Thomas Hunt Morgan (1910) noticed that eye color in fruit flies and wing size always seemed to be inherited together in offspring. – This seemed to go against Mendel’s Law of Independent Assortment Morgan discovered chromosomes are a group of linked genes Law of Independent Assortment was still true, but chromosomes assort independently, not genes.

Gene Linkage Genes located on the same chromosome will NOT sort independently They will be inherited together – Gene Linkage: Tendency of 2 alleles (for different traits) to be inherited together The closer the genes are, the more likely they are to be inherited together

Mendel’s Principle of Independent Assortment? Remember: this refers to genes on separate chromosomes Gene 1 Gene 2

What would cause genes on the same chromosome to be inherited separately? – Crossing over!

Location of Gene on a chromosome is it’s LOCUS

KARYOTYPE AND GENETIC DISORDERS H. Biology

Chromosomes Determine Your Gender Humans have 23 pairs of chromosomes autosomes 22 pairs are autosomes (chromosomes that are NOT involved in making gender) – Also called homologous chromosome pairs 1 pair are sex chromosomes (NOT HOMOLOGOUS; different genes) – The sex chromosomes are “X” and “Y” » “XX” is a girl » “XY” is a boy X Y Y X Egg cells ALWAYS have “X” chromosome

5 µm Pair of homologous chromosomes Centromere Sister chromatids A karyotype – an ordered, visual representation of ALL chromosomes in a human cell – Arranged according to homologous pairs

a REAL view… wow!!!

What is a genetic disorder? What causes a genetic disorder? Caused by an abnormality in an individual’s DNA How do these abnormalities occur? 1)Non-disjunction – Addition or subtraction of chromosome or set of chromosomes 2)Mutations – Change to DNA sequence (gene)

Abnormal Chromosome Number When nondisjunction occurs – Pairs of homologous chromosomes do NOT separate normally during meiosis – Gametes contain 2 copies or NO copies of a particular chromosome What are some disorders caused by non-disjunction? Downs Syndrome – Trisomy 21 Klinefelter’s Syndrome – XXY Turner Syndrome – Monosomy X

What happens when meiosis goes wrong?  NONDISJUNCTION!!!

Figure 15.12a, b Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n  1 n – 1 n + 1n –1 n n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b)

Aneuploidy Results from the fertilization of gametes in which nondisjunction occurred abnormal number Offspring have an abnormal number of a particular chromosome

What are some disorders caused by non- disjunction? Downs Syndrome – Trisomy 21 Kleinfelter’s Syndrome – XXY Edwards Syndrome – Trisomy 18 Patau Syndrome – Trisomy 13 Turner Syndrome – Monosomy X

1.) Trisomy 18-Edwards Syndrome

2.) Trisomy-13: Usually Fatal

3.) Klinefelter - XXY

Klinefelter syndrome – a result of an extra chromosome in a male, producing XXY individuals – 1/2000 live births

XYY individuals

Triple X Syndrome XXX individuals Females Healthy Same phenotype as XX individuals 1/1000 live births

4.) Turner Syndrome

Turner syndrome – Is the result of monosomy X, producing an X0 karyotype – 1/5000 live births – Only known monosomy in humans – Normal intelligence – Sterile

5.) Trisomy 21

Human Disorders Due to Chromosomal Alterations Down syndrome – Is usually the result of an extra chromosome 21 trisomy 21

What are some disorders caused by mutations? Sickle cell anemia (recessive) Cystic fibrosis (recessive) Maple syrup urine disorder (recessive) Congenital hypothyroidism (recessive) Huntington’s Disease (dominant)

Cystic Fibrosis Example of recessive disorder Affect mostly people of European descent Symptoms – Mucus buildup in the some internal organs – Abnormal absorption of nutrients in the small intestine

Sickle Cell Anemia o Another recessive disorder o Affects one out of 400 African-Americans o Hemoglobin is the O2 carrying protein in red blood cells o SCA is an inherited blood disease where hemoglobin clumps together  causes red blood cells to stiffen and curl into a sickle/crescent shape o Because of this, the RBC cannot transport O2 effectively Symptoms Physical weakness, pain, organ damage, and even paralysis

Human Chromosomal Disorders Recessive Disorders Cystic Fibrosis – Excess mucus in lungs, digestive tract, liver – More susceptible to pneumonia, infection – Early death ~ average age of 37 Tay-Sachs – Lipid accumulation in brain cells, mental retardation – Blindness, death in childhood Dominant Disorder Huntington’s disease – Deterioration of nervous system, uncontrollable movements – Develops in middle age, death soon after 32

Dominantly Inherited Disorders Some human disorders – Are due to dominant alleles  only need 1 dominant allele Example is achondroplasia – Form of dwarfism  lethal when homozygous for the dominant allele

Another Dominant Disorder Huntington’s disease (HD) – degenerative disease of nervous system – No obvious phenotypic effects until about 35 to 40 years of age HD Normal