Heredity Unit 3 Chapter 29 a Heredity Who

Heredity Unit 3 Chapter 29 a

Heredity �Who we are is guided by the genebearing chromosomes we receive from our parents in egg and sperm. �Segments of DNA called genes are blueprints for proteins, many which are enzymes, that dictate the synthesis of all of our body’s molecules.

Heredity �Genes are expressed in our hair color, sex, blood type and so on �However, these genes are influenced by other genes and by environmental influences

Genetics �Genetics is the study of the mechanism of heredity �Genes = give birth to �Nuclei of all human cells (except gametes) contain 46 chromosomes (or 23 pair) �Sex chromosomes determine the genetic sex (XX = female, XY = male)

Genetics �Karyotype – the diploid chromosomal complement displayed in homologous pairs – a picture of our genome �Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal

Alleles �Alleles - Matched genes at the same locus on homologous chromosomes �Homozygous – two alleles controlling a single trait are the same �Heterozygous – the two alleles for a trait are different

Alleles �Dominant – an allele masks or suppresses the expression of its partner – represented by a capital letter �Recessive – the allele that is masked or suppressed – represented by a lower case letter

Alleles & Genotype �AA = both alleles dominate – homozygous dominant �Aa = one dominant allele and one recessive allele = heterozygous �aa = both alleles recessive – homozygous recessive

Genotype and Phenotype �Genotype – the genetic makeup �Phenotype – the way one’s genotype is expressed

Segregation and Independent Assortment �Chromosomes are randomly distributed to daughter cells �Members of the allele pair for each trait are segregated during meiosis �Alleles on different pairs of homologous chromosomes are distributed independently

Segregation and Independent Assortment �The number of different types of gametes can be calculated by this formula: 2 n, where n is the number of homologous pairs

Segregation and Independent Assortment �In a man’s testes, the number of gamete types that can be produced based on independent assortment is 223, which equals 8. 5 million possibilities

Independent Assortment Figure 29. 2

Crossover �Homologous chromosomes synapse in meiosis I �One chromosome segment exchanges positions with its homologous counterpart �Genetic information is exchanged between homologous chromosomes �Two recombinant chromosomes are formed

Crossover Figure 29. 3

Crossover Figure 29. 3

Random Fertilization �A single egg is fertilized by a single sperm in a random manner �Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8. 5 million 8. 5 million) zygote possibilities

Dominant-Recessive Inheritance �Reflects the interaction of dominant and recessive alleles �Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype

Dominant-Recessive Inheritance �Example: probability of different offspring from mating two heterozygous parents T = tongue roller and t = cannot roll tongue

Figure 29. 4

Dominant-Recessive Inheritance �Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease �Examples of recessive conditions: albinism, cystic fibrosis, and Tay. Sachs disease �Carriers – heterozygotes who do not express a trait but can pass it on to their offspring

�Now try some Punnet Square problems on you own!

Quiz next time! Study guide check Pages 713 -718 (6 points)

Heredity Unit 3 Chapter 29 b

Incomplete Dominance �Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive

Incomplete Dominance �Sickling gene is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s) SS = normal Hb is made Ss = sickle-cell trait (both aberrant and normal Hb is made) ss = sickle-cell anemia (only aberrant Hb is made)

Multiple-Allele Inheritance �Genes that exhibit more than two alternate alleles �ABO blood grouping is an example �Three alleles (IA, IB, i) determine the ABO blood type in humans �IA and IB are codominant (both are expressed if present), and i is recessive

ABO Blood Groups Table 29. 2

Sex-Linked Inheritance �Inherited traits determined by genes on the sex chromosomes �X chromosomes bear over 2500 genes; Y chromosomes carry about 15 genes

Sex-Linked Inheritance �X-linked genes are: �Found only on the X chromosome �Typically passed from mothers to sons �Never masked or damped in males since there is no Y counterpart

Polygene Inheritance �Depends on several different gene pairs at different loci acting in tandem �Results in continuous phenotypic variation between two extremes �Examples: skin color, eye color, and height

Polygenic Inheritance of Skin Color �Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc) �The first generation offspring each have three “units” of darkness (intermediate pigmentation) �The second generation offspring have a wide variation in possible pigmentations

Polygenic Inheritance of Skin Color Figure 29. 5

Environmental Influence on Gene Expression �Phenocopies – environmentally produced phenotypes that mimic mutations

Environmental Influence on Gene Expression �Environmental factors can influence genetic expression after birth �Poor nutrition can effect brain growth, body development, and height �Childhood hormonal deficits can lead to abnormal skeletal growth

Genomic Imprinting �The same allele can have different effects depending upon the source parent �Deletions in chromosome 15 result in: �Prader-Willi syndrome if inherited from the father �Angelman syndrome if inherited from the mother

Genomic Imprinting �During gametogenesis, certain genes are methylated and tagged as either maternal or paternal �Developing embryos “read” these tags and express one version or the other

Extrachromosomal (Mitochondrial) Inheritance �Some genes are in the mitochondria �All mitochondrial genes are transmitted by the mother �Unusual muscle disorders and neurological problems have been linked to these genes

Heredity Unit 3 Chapter 29 c

Genetic Screening, Counseling, and Therapy �Newborn infants are screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and PKU

Genetic Screening, Counseling, and Therapy �Genetic screening alerts new parents that treatment may be necessary for the well-being of their infant �Example: a woman pregnant for the first time at age 35 may want to know if her baby has trisomy-21 (Down syndrome)

Carrier Recognition �Identification of the heterozygote state for a given trait �Two major avenues are used to identify carriers: pedigrees and blood tests

Carrier Recognition �Pedigrees trace a particular genetic trait through several generations; helps to predict the future �Blood tests and DNA probes can detect the presence of unexpressed recessive genes �Sickling, Tay-Sachs, and cystic fibrosis genes can be identified by such tests

Pedigree Analysis Figure 29. 6

Fetal Testing �Is used when there is a known risk of a genetic disorder �Amniocentesis – amniotic fluid is withdrawn after the 14 th week and sloughed fetal cells are examined for genetic abnormalities �Chorionic villi sampling (CVS) – chorionic villi are sampled and karyotyped for genetic abnormalities

Fetal Testing Figure 29. 7

Human Gene Therapy �Genetic engineering has the potential to replace a defective gene �Defective cells can be infected with a genetically engineered virus containing a functional gene �The patient’s cells can be directly injected with “corrected” DNA

Quiz next time! Study guide check Pages 719 – 726 (8 pts)