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MENDELIAN GENETICS One Factor Crosses • LEARNING GOALS: I can… …understand how chromosomes are arranged during meiosis …use the terms dominant and recessive & homozygous and heterozygous correctly …use a Punnett square to determine the possibility and probability of gene combinations between parents and the resulting traits I. Determining Genotypes A. Every living organism has a g e n o t y p e that determines their p h e n o t y p e . Genotype – a n i n d i v i d u a l ’s c o l l e c t i o n o f g e n e s Phenotype – a n i n d i v i d u a l ’s o b s e r v a b l e t ra i t s 1. An organism’s genotype comes from t h e i r p a r e n t s a n d i s found in the nuclei (eukaryotic) of their cells (DNA) 2. An organism’s phenotype then comes from t h e i r g e n o t y p e “The genes of an organism determines the organism’s phenotype” B. For almost every gene there is more than one kind or form, called a l l e l e s . Allele – a va r i a nt fo r m o f a ge n e 1. You must remember that alleles c o d e f o r t h e s a m e trait (for example: eye color) but come in d i f f e r e n t f o r m s (blue e y e c o l o r o r brown e y e color) 2. Alleles can either be d o m i n a n t or r e c e s s i v e . Dominant – a t r a i t t h a t i s p h e n o t y p i c a l l y e x p r e s s e d whenever one copy of the allele is inherited Recessive – a t r a i t t h a t i s p h e n o t y p i c a l l y e x p r e s s e d only when two copies of the allele are inherited C. Organisms can reproduce s exu a l l y or a s exu a l l y . 1. 2. Organisms that reproduce asexually will be genetically identical to their parents UNLESS their DNA is mutated Organisms that reproduce sexually will be d i f f e r e n t from either parent because their genotype is a combination from two d i f f e r e n t p e o p l e P LU S m u t a t i o n s c a n o c c u r. D. During meiosis ga m ete s a re m a d e w i t h h a l f t h e n u m b e r o f c h ro m o s o m e s o f s o m at i c cells (haploid) 1. Because of the LAW OF INDEPENDENT ASSORTMENT o n l y genes on the same chromosome are inherited t o g e t h e r. 2. Because of the PRINCIPLE OF SEGREGATION c h r o m o s o m e s s e p a r a t e a n d a r r a n g e t h e m s e l v e s r a n d o m l y. E. When fertilization occurs t wo ga m ete s w i l l f u s e ra n d o m l y w i t h a c o m b i n at i o n o f a l l e l e s 1. If an organism inherits two of the same allele for one trait they are said to be h o m o z y g o u s . 2. If an organism inherits two different alleles for one trait they are said to h e t e r o z y g o u s . II. Using Punnett squares to determine gene combinations A. When biologists choose symbols to represent genes they usually take the first letter of the dominant trait. The upper case form of that letter represents the dominant allele the lower case form of that letter represents the recessive allele. • Practice determining symbols for traits: green is dominant over white G = green g=white round is dominant over wrinkled R = round r=wrinkled tall is dominant over short T = tall t = short brown is dominant over blue B = brown b = blue • The symbols can then be used with a Punnett square to determine gene combinations Red flowers are dominant over white flowers PARENT #1 = homozygous red PARENT #2 = homozygous white (Follow the example on the board) Use the information provided and a Punnett square to determine the possible gene combination from the following one factor crosses. Then, determine the probability of an offspring having a given gene combination. Finally, determine the probability of an offspring have the dominant or recessive trait. For your information: smooth is dominant over wrinkled brown is dominant over blue brown is dominant over blonde tall is dominant over short 1. homozygous brown X homozygous blonde 2. homozygous brown X homozygous brown 3. heterozygous tall X heterozygous tall